diff --git a/assets/js/backend_admins.js b/assets/js/backend_admins.js
index fb1c4100..b553e7f2 100644
--- a/assets/js/backend_admins.js
+++ b/assets/js/backend_admins.js
@@ -90,12 +90,12 @@ window.BackendAdmins = window.BackendAdmins || {};
 
             $.post(url, data).done(function (response) {
                 if (response.is_valid === 'false') {
-                    $input.closest('.form-group').addClass('has-error');
+                    $input.addClass('is-invalid');
                     $input.attr('already-exists', 'true');
                     $input.parents().eq(3).find('.form-message').text(EALang.username_already_exists);
                     $input.parents().eq(3).find('.form-message').show();
                 } else {
-                    $input.closest('.form-group').removeClass('has-error');
+                    $input.removeClass('is-invalid');
                     $input.attr('already-exists', 'false');
                     if ($input.parents().eq(3).find('.form-message').text() === EALang.username_already_exists) {
                         $input.parents().eq(3).find('.form-message').hide();
diff --git a/assets/js/backend_admins_helper.js b/assets/js/backend_admins_helper.js
index 3651a4d5..284b9896 100644
--- a/assets/js/backend_admins_helper.js
+++ b/assets/js/backend_admins_helper.js
@@ -276,7 +276,7 @@
      * @return {Boolean} Returns the validation result.
      */
     AdminsHelper.prototype.validate = function () {
-        $('#admins .has-error').removeClass('has-error');
+        $('#admins .is-invalid').removeClass('is-invalid');
 
         try {
             // Validate required fields.
@@ -284,7 +284,7 @@
 
             $('#admins .required').each(function (index, requiredField) {
                 if (!$(requiredField).val()) {
-                    $(requiredField).closest('.form-group').addClass('has-error');
+                    $(requiredField).addClass('is-invalid');
                     missingRequired = true;
                 }
             });
@@ -295,7 +295,7 @@
 
             // Validate passwords.
             if ($('#admin-password').val() !== $('#admin-password-confirm').val()) {
-                $('#admin-password, #admin-password-confirm').closest('.form-group').addClass('has-error');
+                $('#admin-password, #admin-password-confirm').addClass('is-invalid');
                 throw new Error(EALang.passwords_mismatch);
             }
 
@@ -303,19 +303,19 @@
                 $('#admin-password').val().length < BackendAdmins.MIN_PASSWORD_LENGTH &&
                 $('#admin-password').val() !== ''
             ) {
-                $('#admin-password, #admin-password-confirm').closest('.form-group').addClass('has-error');
+                $('#admin-password, #admin-password-confirm').addClass('is-invalid');
                 throw new Error(EALang.password_length_notice.replace('$number', BackendAdmins.MIN_PASSWORD_LENGTH));
             }
 
             // Validate user email.
             if (!GeneralFunctions.validateEmail($('#admin-email').val())) {
-                $('#admin-email').closest('.form-group').addClass('has-error');
+                $('#admin-email').addClass('is-invalid');
                 throw new Error(EALang.invalid_email);
             }
 
             // Check if username exists
             if ($('#admin-username').attr('already-exists') === 'true') {
-                $('#admin-username').closest('.form-group').addClass('has-error');
+                $('#admin-username').addClass('is-invalid');
                 throw new Error(EALang.username_already_exists);
             }
 
@@ -341,7 +341,7 @@
         $('#admins .record-details #admin-timezone').val('UTC');
         $('#edit-admin, #delete-admin').prop('disabled', true);
 
-        $('#admins .has-error').removeClass('has-error');
+        $('#admins .is-invalid').removeClass('is-invalid');
         $('#admins .form-message').hide();
     };
 
diff --git a/assets/js/backend_calendar_appointments_modal.js b/assets/js/backend_calendar_appointments_modal.js
index 3d899c90..40d8f7c3 100755
--- a/assets/js/backend_calendar_appointments_modal.js
+++ b/assets/js/backend_calendar_appointments_modal.js
@@ -578,7 +578,7 @@ window.BackendCalendarAppointmentsModal = window.BackendCalendarAppointmentsModa
         var $dialog = $('#manage-appointment');
 
         // Reset previous validation css formatting.
-        $dialog.find('.has-error').removeClass('has-error');
+        $dialog.find('.is-invalid').removeClass('is-invalid');
         $dialog.find('.modal-message').addClass('d-none');
 
         try {
@@ -587,7 +587,7 @@ window.BackendCalendarAppointmentsModal = window.BackendCalendarAppointmentsModa
 
             $dialog.find('.required').each(function (index, requiredField) {
                 if ($(requiredField).val() === '' || $(requiredField).val() === null) {
-                    $(requiredField).closest('.form-group').addClass('has-error');
+                    $(requiredField).addClass('is-invalid');
                     missingRequiredField = true;
                 }
             });
@@ -598,7 +598,7 @@ window.BackendCalendarAppointmentsModal = window.BackendCalendarAppointmentsModa
 
             // Check email address.
             if (!GeneralFunctions.validateEmail($dialog.find('#email').val())) {
-                $dialog.find('#email').closest('.form-group').addClass('has-error');
+                $dialog.find('#email').addClass('is-invalid');
                 throw new Error(EALang.invalid_email);
             }
 
@@ -606,7 +606,7 @@ window.BackendCalendarAppointmentsModal = window.BackendCalendarAppointmentsModa
             var start = $('#start-datetime').datetimepicker('getDate');
             var end = $('#end-datetime').datetimepicker('getDate');
             if (start > end) {
-                $dialog.find('#start-datetime, #end-datetime').closest('.form-group').addClass('has-error');
+                $dialog.find('#start-datetime, #end-datetime').addClass('is-invalid');
                 throw new Error(EALang.start_date_before_end_error);
             }
 
diff --git a/assets/js/backend_calendar_unavailability_events_modal.js b/assets/js/backend_calendar_unavailability_events_modal.js
index 90607992..d56f72d4 100755
--- a/assets/js/backend_calendar_unavailability_events_modal.js
+++ b/assets/js/backend_calendar_unavailability_events_modal.js
@@ -30,19 +30,19 @@ window.BackendCalendarUnavailabilityEventsModal = window.BackendCalendarUnavaila
         $('#manage-unavailable #save-unavailable').on('click', function () {
             var $dialog = $('#manage-unavailable');
             $dialog.find('.modal-message').addClass('d-none');
-            $dialog.find('.has-error').removeClass('has-error');
+            $dialog.find('.is-invalid').removeClass('is-invalid');
 
             var start = $dialog.find('#unavailable-start').datetimepicker('getDate');
 
             if (!start) {
-                $dialog.find('#unavailable-start').closest('.form-group').addClass('has-error');
+                $dialog.find('#unavailable-start').addClass('is-invalid');
                 return;
             }
 
             var end = Date.parse($dialog.find('#unavailable-end').datetimepicker('getDate'));
 
             if (!end) {
-                $dialog.find('#unavailable-end').closest('.form-group').addClass('has-error');
+                $dialog.find('#unavailable-end').addClass('is-invalid');
                 return;
             }
 
@@ -54,7 +54,7 @@ window.BackendCalendarUnavailabilityEventsModal = window.BackendCalendarUnavaila
                     .addClass('alert-danger')
                     .removeClass('d-none');
 
-                $dialog.find('#unavailable-start, #unavailable-end').closest('.form-group').addClass('has-error');
+                $dialog.find('#unavailable-start, #unavailable-end').addClass('is-invalid');
                 return;
             }
 
diff --git a/assets/js/backend_calendar_working_plan_exceptions_modal.js b/assets/js/backend_calendar_working_plan_exceptions_modal.js
index 233737be..f54f67bb 100644
--- a/assets/js/backend_calendar_working_plan_exceptions_modal.js
+++ b/assets/js/backend_calendar_working_plan_exceptions_modal.js
@@ -30,26 +30,26 @@ window.BackendCalendarWorkingPlanExceptionsModal = window.BackendCalendarWorking
         $('#manage-working-plan-exceptions #save-working-plan-exception').on('click', function () {
             $('#manage-working-plan-exceptions .modal-message').prop('hidden', true);
 
-            $('#manage-working-plan-exceptions').find('.has-error').removeClass('has-error');
+            $('#manage-working-plan-exceptions').find('.is-invalid').removeClass('is-invalid');
 
             var date = $('#working-plan-exception-date').datetimepicker('getDate');
 
             if (!date) {
-                $('#working-plan-exception-date').closest('.form-group').addClass('has-error');
+                $('#working-plan-exception-date').addClass('is-invalid');
                 return;
             }
 
             var start = $('#working-plan-exception-start').datetimepicker('getDate');
 
             if (!start) {
-                $('#working-plan-exception-start').closest('.form-group').addClass('has-error');
+                $('#working-plan-exception-start').addClass('is-invalid');
                 return;
             }
 
             var end = Date.parse($('#working-plan-exception-end').datetimepicker('getDate'));
 
             if (!end) {
-                $('#working-plan-exception-end').closest('.form-group').addClass('has-error');
+                $('#working-plan-exception-end').addClass('is-invalid');
                 return;
             }
 
@@ -60,8 +60,8 @@ window.BackendCalendarWorkingPlanExceptionsModal = window.BackendCalendarWorking
                     .addClass('alert-danger')
                     .prop('hidden', false);
 
-                $('#working-plan-exception-start').addClass('has-error');
-                $('#working-plan-exception-end').addClass('has-error');
+                $('#working-plan-exception-start').addClass('is-invalid');
+                $('#working-plan-exception-end').addClass('is-invalid');
                 return;
             }
 
diff --git a/assets/js/backend_categories_helper.js b/assets/js/backend_categories_helper.js
index b9ce9be9..727c5d88 100644
--- a/assets/js/backend_categories_helper.js
+++ b/assets/js/backend_categories_helper.js
@@ -288,7 +288,7 @@
      * @return {Boolean} Returns the validation result.
      */
     CategoriesHelper.prototype.validate = function () {
-        $('#categories .has-error').removeClass('has-error');
+        $('#categories .is-invalid').removeClass('is-invalid');
         $('#categories .form-message').removeClass('alert-danger').hide();
 
         try {
@@ -296,7 +296,7 @@
 
             $('#categories .required').each(function (index, requiredField) {
                 if (!$(requiredField).val()) {
-                    $(requiredField).closest('.form-group').addClass('has-error');
+                    $(requiredField).addClass('is-invalid');
                     missingRequired = true;
                 }
             });
@@ -325,7 +325,7 @@
         $('#categories .record-details').find('input, select, textarea').val('').prop('disabled', true);
         $('#edit-category, #delete-category').prop('disabled', true);
 
-        $('#categories .record-details .has-error').removeClass('has-error');
+        $('#categories .record-details .is-invalid').removeClass('is-invalid');
         $('#categories .record-details .form-message').hide();
     };
 
diff --git a/assets/js/backend_customers_helper.js b/assets/js/backend_customers_helper.js
index 92235489..aa98dd71 100644
--- a/assets/js/backend_customers_helper.js
+++ b/assets/js/backend_customers_helper.js
@@ -212,7 +212,7 @@
      */
     CustomersHelper.prototype.validate = function () {
         $('#form-message').removeClass('alert-danger').hide();
-        $('.has-error').removeClass('has-error');
+        $('.is-invalid').removeClass('is-invalid');
 
         try {
             // Validate required fields.
@@ -220,7 +220,7 @@
 
             $('.required').each(function (index, requiredField) {
                 if ($(requiredField).val() === '') {
-                    $(requiredField).closest('.form-group').addClass('has-error');
+                    $(requiredField).addClass('is-invalid');
                     missingRequired = true;
                 }
             });
@@ -231,7 +231,7 @@
 
             // Validate email address.
             if (!GeneralFunctions.validateEmail($('#email').val())) {
-                $('#email').closest('.form-group').addClass('has-error');
+                $('#email').addClass('is-invalid');
                 throw new Error(EALang.invalid_email);
             }
 
@@ -256,7 +256,7 @@
         $('#add-edit-delete-group').show();
         $('#save-cancel-group').hide();
 
-        $('.record-details .has-error').removeClass('has-error');
+        $('.record-details .is-invalid').removeClass('is-invalid');
         $('.record-details #form-message').hide();
 
         $('#filter-customers button').prop('disabled', false);
diff --git a/assets/js/backend_providers.js b/assets/js/backend_providers.js
index 53b3e135..111a9b7e 100644
--- a/assets/js/backend_providers.js
+++ b/assets/js/backend_providers.js
@@ -126,12 +126,12 @@ window.BackendProviders = window.BackendProviders || {};
 
             $.post(url, data).done(function (response) {
                 if (response.is_valid === 'false') {
-                    $input.closest('.form-group').addClass('has-error');
+                    $input.addClass('is-invalid');
                     $input.attr('already-exists', 'true');
                     $input.parents().eq(3).find('.form-message').text(EALang.username_already_exists);
                     $input.parents().eq(3).find('.form-message').show();
                 } else {
-                    $input.closest('.form-group').removeClass('has-error');
+                    $input.removeClass('is-invalid');
                     $input.attr('already-exists', 'false');
                     if ($input.parents().eq(3).find('.form-message').text() === EALang.username_already_exists) {
                         $input.parents().eq(3).find('.form-message').hide();
diff --git a/assets/js/backend_providers_helper.js b/assets/js/backend_providers_helper.js
index ecbf7569..591b5525 100755
--- a/assets/js/backend_providers_helper.js
+++ b/assets/js/backend_providers_helper.js
@@ -322,7 +322,7 @@
      * @return {Boolean} Returns the validation result.
      */
     ProvidersHelper.prototype.validate = function () {
-        $('#providers .has-error').removeClass('has-error');
+        $('#providers .is-invalid').removeClass('is-invalid');
         $('#providers .form-message').removeClass('alert-danger').hide();
 
         try {
@@ -330,7 +330,7 @@
             var missingRequired = false;
             $('#providers .required').each(function (index, requiredField) {
                 if (!$(requiredField).val()) {
-                    $(requiredField).closest('.form-group').addClass('has-error');
+                    $(requiredField).addClass('is-invalid');
                     missingRequired = true;
                 }
             });
@@ -340,7 +340,7 @@
 
             // Validate passwords.
             if ($('#provider-password').val() !== $('#provider-password-confirm').val()) {
-                $('#provider-password, #provider-password-confirm').closest('.form-group').addClass('has-error');
+                $('#provider-password, #provider-password-confirm').addClass('is-invalid');
                 throw new Error(EALang.passwords_mismatch);
             }
 
@@ -348,19 +348,19 @@
                 $('#provider-password').val().length < BackendProviders.MIN_PASSWORD_LENGTH &&
                 $('#provider-password').val() !== ''
             ) {
-                $('#provider-password, #provider-password-confirm').closest('.form-group').addClass('has-error');
+                $('#provider-password, #provider-password-confirm').addClass('is-invalid');
                 throw new Error(EALang.password_length_notice.replace('$number', BackendProviders.MIN_PASSWORD_LENGTH));
             }
 
             // Validate user email.
             if (!GeneralFunctions.validateEmail($('#provider-email').val())) {
-                $('#provider-email').closest('.form-group').addClass('has-error');
+                $('#provider-email').addClass('is-invalid');
                 throw new Error(EALang.invalid_email);
             }
 
             // Check if username exists
             if ($('#provider-username').attr('already-exists') === 'true') {
-                $('#provider-username').closest('.form-group').addClass('has-error');
+                $('#provider-username').addClass('is-invalid');
                 throw new Error(EALang.username_already_exists);
             }
 
@@ -394,7 +394,7 @@
             .find('.edit-working-plan-exception, .delete-working-plan-exception')
             .prop('disabled', true);
 
-        $('#providers .record-details .has-error').removeClass('has-error');
+        $('#providers .record-details .is-invalid').removeClass('is-invalid');
         $('#providers .record-details .form-message').hide();
 
         $('#edit-provider, #delete-provider').prop('disabled', true);
diff --git a/assets/js/backend_secretaries.js b/assets/js/backend_secretaries.js
index 59d4640b..17bd9eeb 100644
--- a/assets/js/backend_secretaries.js
+++ b/assets/js/backend_secretaries.js
@@ -113,12 +113,12 @@ window.BackendSecretaries = window.BackendSecretaries || {};
 
             $.post(url, data).done(function (response) {
                 if (response.is_valid === 'false') {
-                    $input.closest('.form-group').addClass('has-error');
+                    $input.addClass('is-invalid');
                     $input.attr('already-exists', 'true');
                     $input.parents().eq(3).find('.form-message').text(EALang.username_already_exists);
                     $input.parents().eq(3).find('.form-message').show();
                 } else {
-                    $input.closest('.form-group').removeClass('has-error');
+                    $input.removeClass('is-invalid');
                     $input.attr('already-exists', 'false');
                     if ($input.parents().eq(3).find('.form-message').text() === EALang.username_already_exists) {
                         $input.parents().eq(3).find('.form-message').hide();
diff --git a/assets/js/backend_secretaries_helper.js b/assets/js/backend_secretaries_helper.js
index 8a221387..557c8030 100644
--- a/assets/js/backend_secretaries_helper.js
+++ b/assets/js/backend_secretaries_helper.js
@@ -289,7 +289,7 @@
      * @return {Boolean} Returns the validation result.
      */
     SecretariesHelper.prototype.validate = function () {
-        $('#secretaries .has-error').removeClass('has-error');
+        $('#secretaries .is-invalid').removeClass('is-invalid');
         $('#secretaries .form-message').removeClass('alert-danger');
 
         try {
@@ -297,7 +297,7 @@
             var missingRequired = false;
             $('#secretaries .required').each(function (index, requiredField) {
                 if (!$(requiredField).val()) {
-                    $(requiredField).closest('.form-group').addClass('has-error');
+                    $(requiredField).addClass('is-invalid');
                     missingRequired = true;
                 }
             });
@@ -307,7 +307,7 @@
 
             // Validate passwords.
             if ($('#secretary-password').val() !== $('#secretary-password-confirm').val()) {
-                $('#secretary-password, #secretary-password-confirm').closest('.form-group').addClass('has-error');
+                $('#secretary-password, #secretary-password-confirm').addClass('is-invalid');
                 throw new Error('Passwords mismatch!');
             }
 
@@ -315,19 +315,19 @@
                 $('#secretary-password').val().length < BackendSecretaries.MIN_PASSWORD_LENGTH &&
                 $('#secretary-password').val() !== ''
             ) {
-                $('#secretary-password, #secretary-password-confirm').closest('.form-group').addClass('has-error');
+                $('#secretary-password, #secretary-password-confirm').addClass('is-invalid');
                 throw new Error('Password must be at least ' + BackendSecretaries.MIN_PASSWORD_LENGTH + ' characters long.');
             }
 
             // Validate user email.
             if (!GeneralFunctions.validateEmail($('#secretary-email').val())) {
-                $('#secretary-email').closest('.form-group').addClass('has-error');
+                $('#secretary-email').addClass('is-invalid');
                 throw new Error('Invalid email address!');
             }
 
             // Check if username exists
             if ($('#secretary-username').attr('already-exists') === 'true') {
-                $('#secretary-username').closest('.form-group').addClass('has-error');
+                $('#secretary-username').addClass('is-invalid');
                 throw new Error('Username already exists.');
             }
 
@@ -354,7 +354,7 @@
         $('#edit-secretary, #delete-secretary').prop('disabled', true);
         $('#secretaries .form-message').hide();
         $('#secretary-providers input:checkbox').prop('checked', false);
-        $('#secretaries .has-error').removeClass('has-error');
+        $('#secretaries .is-invalid').removeClass('is-invalid');
     };
 
     /**
diff --git a/assets/js/backend_services_helper.js b/assets/js/backend_services_helper.js
index 11f00521..289c0dc2 100644
--- a/assets/js/backend_services_helper.js
+++ b/assets/js/backend_services_helper.js
@@ -255,7 +255,7 @@
      * @return {Boolean} Returns the validation result.
      */
     ServicesHelper.prototype.validate = function () {
-        $('#services .has-error').removeClass('has-error');
+        $('#services .is-invalid').removeClass('is-invalid');
         $('#services .form-message').removeClass('alert-danger').hide();
 
         try {
@@ -264,7 +264,7 @@
 
             $('#services .required').each(function (index, requiredField) {
                 if (!$(requiredField).val()) {
-                    $(requiredField).closest('.form-group').addClass('has-error');
+                    $(requiredField).addClass('is-invalid');
                     missingRequired = true;
                 }
             });
@@ -275,7 +275,7 @@
 
             // Validate the duration.
             if (Number($('#service-duration').val()) < 5) {
-                $('#service-duration').closest('.form-group').addClass('has-error');
+                $('#service-duration').addClass('is-invalid');
                 throw new Error(EALang.invalid_duration);
             }
 
@@ -301,7 +301,7 @@
         $('#services .save-cancel-group').hide();
         $('#edit-service, #delete-service').prop('disabled', true);
 
-        $('#services .record-details .has-error').removeClass('has-error');
+        $('#services .record-details .is-invalid').removeClass('is-invalid');
         $('#services .record-details .form-message').hide();
     };
 
diff --git a/assets/js/backend_settings_business_logic_helper.js b/assets/js/backend_settings_business_logic_helper.js
index 5cf47377..1f234398 100644
--- a/assets/js/backend_settings_business_logic_helper.js
+++ b/assets/js/backend_settings_business_logic_helper.js
@@ -96,14 +96,14 @@
      * @return {Boolean} Returns the validation result.
      */
     SystemSettingsBusinessLogicHelper.prototype.validate = function () {
-        $('#business-logic .has-error').removeClass('has-error');
+        $('#business-logic .is-invalid').removeClass('is-invalid');
 
         try {
             // Validate required fields.
             var missingRequired = false;
             $('#business-logic .required').each(function (index, requiredField) {
                 if (!$(requiredField).val()) {
-                    $(requiredField).closest('.form-group').addClass('has-error');
+                    $(requiredField).addClass('is-invalid');
                     missingRequired = true;
                 }
             });
diff --git a/assets/js/backend_settings_client_form_helper.js b/assets/js/backend_settings_client_form_helper.js
index b07a9e24..27bdfc87 100644
--- a/assets/js/backend_settings_client_form_helper.js
+++ b/assets/js/backend_settings_client_form_helper.js
@@ -135,14 +135,14 @@
      * @return {Boolean} Returns the validation result.
      */
     SystemSettingsClientFormHelper.prototype.validate = function () {
-        $('#client-form .has-error').removeClass('has-error');
+        $('#client-form .is-invalid').removeClass('is-invalid');
 
         try {
             // Validate required fields.
             var missingRequired = false;
             $('#client-form .required').each(function (index, requiredField) {
                 if (!$(requiredField).val()) {
-                    $(requiredField).closest('.form-group').addClass('has-error');
+                    $(requiredField).addClass('is-invalid');
                     missingRequired = true;
                 }
             });
diff --git a/assets/js/backend_settings_current_user_helper.js b/assets/js/backend_settings_current_user_helper.js
index 7a73f729..2fb9b6fb 100644
--- a/assets/js/backend_settings_current_user_helper.js
+++ b/assets/js/backend_settings_current_user_helper.js
@@ -87,14 +87,14 @@
      * @return {Boolean} Returns the validation result.
      */
     SystemSettingsCurrentUserHelper.prototype.validate = function () {
-        $('#current-user .has-error').removeClass('has-error');
+        $('#current-user .is-invalid').removeClass('is-invalid');
 
         try {
             // Validate required fields.
             var missingRequired = false;
             $('#current-user .required').each(function (index, requiredField) {
                 if (!$(requiredField).val()) {
-                    $(requiredField).closest('.form-group').addClass('has-error');
+                    $(requiredField).addClass('is-invalid');
                     missingRequired = true;
                 }
             });
@@ -105,18 +105,18 @@
 
             // Validate passwords (if provided).
             if ($('#password').val() !== $('#retype-password').val()) {
-                $('#password, #retype-password').closest('.form-group').addClass('has-error');
+                $('#password, #retype-password').addClass('is-invalid');
                 throw new Error(EALang.passwords_mismatch);
             }
 
             // Validate user email.
             if (!GeneralFunctions.validateEmail($('#email').val())) {
-                $('#email').closest('.form-group').addClass('has-error');
+                $('#email').addClass('is-invalid');
                 throw new Error(EALang.invalid_email);
             }
 
             if ($('#username').attr('already-exists') === 'true') {
-                $('#username').closest('.form-group').addClass('has-error');
+                $('#username').addClass('is-invalid');
                 throw new Error(EALang.username_already_exists);
             }
 
diff --git a/assets/js/backend_settings_general_helper.js b/assets/js/backend_settings_general_helper.js
index f65d4718..cd5611e9 100644
--- a/assets/js/backend_settings_general_helper.js
+++ b/assets/js/backend_settings_general_helper.js
@@ -95,14 +95,14 @@
      * @return {Boolean} Returns the validation result.
      */
     SystemSettingsGeneralHelper.prototype.validate = function () {
-        $('#general .has-error').removeClass('has-error');
+        $('#general .is-invalid').removeClass('is-invalid');
 
         try {
             // Validate required fields.
             var missingRequired = false;
             $('#general .required').each(function (index, requiredField) {
                 if (!$(requiredField).val()) {
-                    $(requiredField).closest('.form-group').addClass('has-error');
+                    $(requiredField).addClass('is-invalid');
                     missingRequired = true;
                 }
             });
@@ -113,7 +113,7 @@
 
             // Validate company email address.
             if (!GeneralFunctions.validateEmail($('#company-email').val())) {
-                $('#company-email').closest('.form-group').addClass('has-error');
+                $('#company-email').addClass('is-invalid');
                 throw new Error(EALang.invalid_email);
             }
 
diff --git a/assets/js/backend_settings_legal_contents_helper.js b/assets/js/backend_settings_legal_contents_helper.js
index 1aba6c5f..644567e3 100644
--- a/assets/js/backend_settings_legal_contents_helper.js
+++ b/assets/js/backend_settings_legal_contents_helper.js
@@ -97,14 +97,14 @@
      * @return {Boolean} Returns the validation result.
      */
     SystemSettingsLegalContentsHelper.prototype.validate = function () {
-        $('#legal-contents .has-error').removeClass('has-error');
+        $('#legal-contents .is-invalid').removeClass('is-invalid');
 
         try {
             // Validate required fields.
             var missingRequired = false;
             $('#general .required').each(function (index, requiredField) {
                 if (!$(requiredField).val()) {
-                    $(requiredField).closest('.form-group').addClass('has-error');
+                    $(requiredField).addClass('is-invalid');
                     missingRequired = true;
                 }
             });
diff --git a/assets/js/backend_users.js b/assets/js/backend_users.js
index 7a7811fd..f32c01cf 100644
--- a/assets/js/backend_users.js
+++ b/assets/js/backend_users.js
@@ -219,12 +219,12 @@ window.BackendUsers = window.BackendUsers || {};
 
             $.post(url, data).done(function (response) {
                 if (response.is_valid === 'false') {
-                    $input.closest('.form-group').addClass('has-error');
+                    $input.addClass('is-invalid');
                     $input.attr('already-exists', 'true');
                     $input.parents().eq(3).find('.form-message').text(EALang.username_already_exists);
                     $input.parents().eq(3).find('.form-message').show();
                 } else {
-                    $input.closest('.form-group').removeClass('has-error');
+                    $input.removeClass('is-invalid');
                     $input.attr('already-exists', 'false');
                     if ($input.parents().eq(3).find('.form-message').text() === EALang.username_already_exists) {
                         $input.parents().eq(3).find('.form-message').hide();
diff --git a/assets/js/backend_users_admins.js b/assets/js/backend_users_admins.js
index 94983a10..56a43e77 100644
--- a/assets/js/backend_users_admins.js
+++ b/assets/js/backend_users_admins.js
@@ -276,7 +276,7 @@
      * @return {Boolean} Returns the validation result.
      */
     AdminsHelper.prototype.validate = function () {
-        $('#admins .has-error').removeClass('has-error');
+        $('#admins .is-invalid').removeClass('is-invalid');
 
         try {
             // Validate required fields.
@@ -284,7 +284,7 @@
 
             $('#admins .required').each(function (index, requiredField) {
                 if (!$(requiredField).val()) {
-                    $(requiredField).closest('.form-group').addClass('has-error');
+                    $(requiredField).addClass('is-invalid');
                     missingRequired = true;
                 }
             });
@@ -295,7 +295,7 @@
 
             // Validate passwords.
             if ($('#admin-password').val() !== $('#admin-password-confirm').val()) {
-                $('#admin-password, #admin-password-confirm').closest('.form-group').addClass('has-error');
+                $('#admin-password, #admin-password-confirm').addClass('is-invalid');
                 throw new Error(EALang.passwords_mismatch);
             }
 
@@ -303,19 +303,19 @@
                 $('#admin-password').val().length < BackendUsers.MIN_PASSWORD_LENGTH &&
                 $('#admin-password').val() !== ''
             ) {
-                $('#admin-password, #admin-password-confirm').closest('.form-group').addClass('has-error');
+                $('#admin-password, #admin-password-confirm').addClass('is-invalid');
                 throw new Error(EALang.password_length_notice.replace('$number', BackendUsers.MIN_PASSWORD_LENGTH));
             }
 
             // Validate user email.
             if (!GeneralFunctions.validateEmail($('#admin-email').val())) {
-                $('#admin-email').closest('.form-group').addClass('has-error');
+                $('#admin-email').addClass('is-invalid');
                 throw new Error(EALang.invalid_email);
             }
 
             // Check if username exists
             if ($('#admin-username').attr('already-exists') === 'true') {
-                $('#admin-username').closest('.form-group').addClass('has-error');
+                $('#admin-username').addClass('is-invalid');
                 throw new Error(EALang.username_already_exists);
             }
 
@@ -341,7 +341,7 @@
         $('#admins .record-details #admin-timezone').val('UTC');
         $('#edit-admin, #delete-admin').prop('disabled', true);
 
-        $('#admins .has-error').removeClass('has-error');
+        $('#admins .is-invalid').removeClass('is-invalid');
         $('#admins .form-message').hide();
     };
 
diff --git a/assets/js/backend_users_providers.js b/assets/js/backend_users_providers.js
index 19171804..9681fe9a 100755
--- a/assets/js/backend_users_providers.js
+++ b/assets/js/backend_users_providers.js
@@ -321,7 +321,7 @@
      * @return {Boolean} Returns the validation result.
      */
     ProvidersHelper.prototype.validate = function () {
-        $('#providers .has-error').removeClass('has-error');
+        $('#providers .is-invalid').removeClass('is-invalid');
         $('#providers .form-message').removeClass('alert-danger').hide();
 
         try {
@@ -329,7 +329,7 @@
             var missingRequired = false;
             $('#providers .required').each(function (index, requiredField) {
                 if (!$(requiredField).val()) {
-                    $(requiredField).closest('.form-group').addClass('has-error');
+                    $(requiredField).addClass('is-invalid');
                     missingRequired = true;
                 }
             });
@@ -339,7 +339,7 @@
 
             // Validate passwords.
             if ($('#provider-password').val() !== $('#provider-password-confirm').val()) {
-                $('#provider-password, #provider-password-confirm').closest('.form-group').addClass('has-error');
+                $('#provider-password, #provider-password-confirm').addClass('is-invalid');
                 throw new Error(EALang.passwords_mismatch);
             }
 
@@ -347,19 +347,19 @@
                 $('#provider-password').val().length < BackendUsers.MIN_PASSWORD_LENGTH &&
                 $('#provider-password').val() !== ''
             ) {
-                $('#provider-password, #provider-password-confirm').closest('.form-group').addClass('has-error');
+                $('#provider-password, #provider-password-confirm').addClass('is-invalid');
                 throw new Error(EALang.password_length_notice.replace('$number', BackendUsers.MIN_PASSWORD_LENGTH));
             }
 
             // Validate user email.
             if (!GeneralFunctions.validateEmail($('#provider-email').val())) {
-                $('#provider-email').closest('.form-group').addClass('has-error');
+                $('#provider-email').addClass('is-invalid');
                 throw new Error(EALang.invalid_email);
             }
 
             // Check if username exists
             if ($('#provider-username').attr('already-exists') === 'true') {
-                $('#provider-username').closest('.form-group').addClass('has-error');
+                $('#provider-username').addClass('is-invalid');
                 throw new Error(EALang.username_already_exists);
             }
 
@@ -393,7 +393,7 @@
             .find('.edit-working-plan-exception, .delete-working-plan-exception')
             .prop('disabled', true);
 
-        $('#providers .record-details .has-error').removeClass('has-error');
+        $('#providers .record-details .is-invalid').removeClass('is-invalid');
         $('#providers .record-details .form-message').hide();
 
         $('#edit-provider, #delete-provider').prop('disabled', true);
diff --git a/assets/js/backend_users_secretaries.js b/assets/js/backend_users_secretaries.js
index 5c7b69e6..0122b614 100644
--- a/assets/js/backend_users_secretaries.js
+++ b/assets/js/backend_users_secretaries.js
@@ -289,7 +289,7 @@
      * @return {Boolean} Returns the validation result.
      */
     SecretariesHelper.prototype.validate = function () {
-        $('#secretaries .has-error').removeClass('has-error');
+        $('#secretaries .is-invalid').removeClass('is-invalid');
         $('#secretaries .form-message').removeClass('alert-danger');
 
         try {
@@ -297,7 +297,7 @@
             var missingRequired = false;
             $('#secretaries .required').each(function (index, requiredField) {
                 if (!$(requiredField).val()) {
-                    $(requiredField).closest('.form-group').addClass('has-error');
+                    $(requiredField).addClass('is-invalid');
                     missingRequired = true;
                 }
             });
@@ -307,7 +307,7 @@
 
             // Validate passwords.
             if ($('#secretary-password').val() !== $('#secretary-password-confirm').val()) {
-                $('#secretary-password, #secretary-password-confirm').closest('.form-group').addClass('has-error');
+                $('#secretary-password, #secretary-password-confirm').addClass('is-invalid');
                 throw new Error('Passwords mismatch!');
             }
 
@@ -315,19 +315,19 @@
                 $('#secretary-password').val().length < BackendUsers.MIN_PASSWORD_LENGTH &&
                 $('#secretary-password').val() !== ''
             ) {
-                $('#secretary-password, #secretary-password-confirm').closest('.form-group').addClass('has-error');
+                $('#secretary-password, #secretary-password-confirm').addClass('is-invalid');
                 throw new Error('Password must be at least ' + BackendUsers.MIN_PASSWORD_LENGTH + ' characters long.');
             }
 
             // Validate user email.
             if (!GeneralFunctions.validateEmail($('#secretary-email').val())) {
-                $('#secretary-email').closest('.form-group').addClass('has-error');
+                $('#secretary-email').addClass('is-invalid');
                 throw new Error('Invalid email address!');
             }
 
             // Check if username exists
             if ($('#secretary-username').attr('already-exists') === 'true') {
-                $('#secretary-username').closest('.form-group').addClass('has-error');
+                $('#secretary-username').addClass('is-invalid');
                 throw new Error('Username already exists.');
             }
 
@@ -354,7 +354,7 @@
         $('#edit-secretary, #delete-secretary').prop('disabled', true);
         $('#secretaries .form-message').hide();
         $('#secretary-providers input:checkbox').prop('checked', false);
-        $('#secretaries .has-error').removeClass('has-error');
+        $('#secretaries .is-invalid').removeClass('is-invalid');
     };
 
     /**
diff --git a/assets/js/frontend_book.js b/assets/js/frontend_book.js
index f7321652..986acd07 100644
--- a/assets/js/frontend_book.js
+++ b/assets/js/frontend_book.js
@@ -501,7 +501,7 @@ window.FrontendBook = window.FrontendBook || {};
      * @return {Boolean} Returns the validation result.
      */
     function validateCustomerForm() {
-        $('#wizard-frame-3 .has-error').removeClass('has-error');
+        $('#wizard-frame-3 .is-invalid').removeClass('is-invalid');
         $('#wizard-frame-3 label.text-danger').removeClass('text-danger');
 
         try {
@@ -509,7 +509,7 @@ window.FrontendBook = window.FrontendBook || {};
             var missingRequiredField = false;
             $('.required').each(function (index, requiredField) {
                 if (!$(requiredField).val()) {
-                    $(requiredField).parents('.form-group').addClass('has-error');
+                    $(requiredField).parents('.form-group').addClass('is-invalid');
                     missingRequiredField = true;
                 }
             });
@@ -531,7 +531,7 @@ window.FrontendBook = window.FrontendBook || {};
 
             // Validate email address.
             if (!GeneralFunctions.validateEmail($('#email').val())) {
-                $('#email').parents('.form-group').addClass('has-error');
+                $('#email').parents('.form-group').addClass('is-invalid');
                 throw new Error(EALang.invalid_email);
             }
 
diff --git a/assets/js/frontend_book_api.js b/assets/js/frontend_book_api.js
index d02202ba..7e5ce907 100755
--- a/assets/js/frontend_book_api.js
+++ b/assets/js/frontend_book_api.js
@@ -151,9 +151,9 @@ window.FrontendBookApi = window.FrontendBookApi || {};
         var $captchaText = $('.captcha-text');
 
         if ($captchaText.length > 0) {
-            $captchaText.closest('.form-group').removeClass('has-error');
+            $captchaText.removeClass('is-invalid');
             if ($captchaText.val() === '') {
-                $captchaText.closest('.form-group').addClass('has-error');
+                $captchaText.addClass('is-invalid');
                 return;
             }
         }
@@ -204,7 +204,7 @@ window.FrontendBookApi = window.FrontendBookApi || {};
 
                     $('.captcha-title button').trigger('click');
 
-                    $captchaText.closest('.form-group').addClass('has-error');
+                    $captchaText.addClass('is-invalid');
 
                     return false;
                 }
diff --git a/assets/js/installation.js b/assets/js/installation.js
index 2dfb0a5d..1bae05f0 100644
--- a/assets/js/installation.js
+++ b/assets/js/installation.js
@@ -68,13 +68,13 @@ $(function () {
     function validate() {
         try {
             $alert.removeClass('alert-danger').prop('hidden', true);
-            $('input').closest('.form-group').removeClass('has-error');
+            $('input').removeClass('is-invalid');
 
             // Check for empty fields.
             var missingRequired = false;
             $('input').each(function (index, field) {
                 if (!$(field).val()) {
-                    $(field).closest('.form-group').addClass('has-error');
+                    $(field).addClass('is-invalid');
                     missingRequired = true;
                 }
             });
@@ -85,25 +85,25 @@ $(function () {
 
             // Validate Passwords
             if ($('#password').val() !== $('#retype-password').val()) {
-                $('#password').closest('.form-group').addClass('has-error');
-                $('#retype-password').closest('.form-group').addClass('has-error');
+                $('#password').addClass('is-invalid');
+                $('#retype-password').addClass('is-invalid');
                 throw new Error('Passwords do not match!');
             }
 
             if ($('#password').val().length < MIN_PASSWORD_LENGTH) {
-                $('#password').closest('.form-group').addClass('has-error');
-                $('#retype-password').closest('.form-group').addClass('has-error');
+                $('#password').addClass('is-invalid');
+                $('#retype-password').addClass('is-invalid');
                 throw new Error('The password must be at least ' + MIN_PASSWORD_LENGTH + ' characters long.');
             }
 
             // Validate Email
             if (!GeneralFunctions.validateEmail($('#email').val())) {
-                $('#email').closest('.form-group').addClass('has-error');
+                $('#email').addClass('is-invalid');
                 throw new Error('The email address is invalid!');
             }
 
             if (!GeneralFunctions.validateEmail($('#company-email').val())) {
-                $('#company-email').closest('.form-group').addClass('has-error');
+                $('#company-email').addClass('is-invalid');
                 throw new Error('The email address is invalid!');
             }