When adding a taxonomy reference field and choosing the vocabulary in the form, the ajax symbol appears and then the save button becomes gray with no function.
From the logs:
LogicException: The database connection is not serializable. This probably means you are serializing an object that has an indirect reference to the database connection. Adjust your code so that is not necessary. Alternatively, look at DependencySerializationTrait as a temporary solution. in Drupal\Core\Database\Connection->__sleep() (line 1920 of /var/www/vhosts/example.com/web/core/lib/Drupal/Core/Database/Connection.php).
#0 [internal function]: Drupal\Core\Database\Connection->__sleep()
#1 /var/www/vhosts/example.com/web/core/lib/Drupal/Component/Serialization/PhpSerialize.php(14): serialize()
#2 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/KeyValueStore/DatabaseStorageExpirable.php(129): Drupal\Component\Serialization\PhpSerialize::encode()
#3 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/KeyValueStore/DatabaseStorageExpirable.php(145): Drupal\Core\KeyValueStore\DatabaseStorageExpirable->doSetWithExpire()
#4 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/Form/FormCache.php(193): Drupal\Core\KeyValueStore\DatabaseStorageExpirable->setWithExpire()
#5 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/Form/FormBuilder.php(463): Drupal\Core\Form\FormCache->setCache()
#6 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/Form/FormBuilder.php(441): Drupal\Core\Form\FormBuilder->setCache()
#7 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/Form/FormBuilder.php(633): Drupal\Core\Form\FormBuilder->rebuildForm()
#8 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/Form/FormBuilder.php(326): Drupal\Core\Form\FormBuilder->processForm()
#9 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/Entity/EntityFormBuilder.php(48): Drupal\Core\Form\FormBuilder->buildForm()
#10 /var/www/vhosts/example.com/web/core/modules/field_ui/src/Controller/FieldConfigAddController.php(62): Drupal\Core\Entity\EntityFormBuilder->getForm()
#11 [internal function]: Drupal\field_ui\Controller\FieldConfigAddController->fieldConfigAddConfigureForm()
#12 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array()
#13 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/Render/Renderer.php(638): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#14 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(121): Drupal\Core\Render\Renderer->executeInRenderContext()
#15 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext()
#16 /var/www/vhosts/ example.comvendor/symfony/http-kernel/HttpKernel.php(181): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#17 /var/www/vhosts/ example.comvendor/symfony/http-kernel/HttpKernel.php(76): Symfony\Component\HttpKernel\HttpKernel->handleRaw()
#18 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/StackMiddleware/Session.php(53): Symfony\Component\HttpKernel\HttpKernel->handle()
#19 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(48): Drupal\Core\StackMiddleware\Session->handle()
#20 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/StackMiddleware/ContentLength.php(28): Drupal\Core\StackMiddleware\KernelPreHandle->handle()
#21 /var/www/vhosts/example.com/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(106): Drupal\Core\StackMiddleware\ContentLength->handle()
#22 /var/www/vhosts/example.com/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(85): Drupal\page_cache\StackMiddleware\PageCache->pass()
#23 /var/www/vhosts/example.com/web/modules/contrib/services/src/StackMiddleware/FormatSetter.php(45): Drupal\page_cache\StackMiddleware\PageCache->handle()
#24 /var/www/vhosts/example.com/web/core/modules/ban/src/BanMiddleware.php(50): Drupal\services\StackMiddleware\FormatSetter->handle()
#25 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(48): Drupal\ban\BanMiddleware->handle()
#26 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(51): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle()
#27 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/StackMiddleware/AjaxPageState.php(36): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle()
#28 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/StackMiddleware/StackedHttpKernel.php(51): Drupal\Core\StackMiddleware\AjaxPageState->handle()
#29 /var/www/vhosts/example.com/web/core/lib/Drupal/Core/DrupalKernel.php(741): Drupal\Core\StackMiddleware\StackedHttpKernel->handle()
#30 /var/www/vhosts/example.com/web/index.php(19): Drupal\Core\DrupalKernel->handle()
#31 {main}
Drupal 10.3.3
php 8.3.11
I don't know. As far as I can see, it appeared after upgrading from 10.3.2
Since others haven't added to this issue, I assume it has something to do with your environment. Can you disable custom/contrib modules one by one and see if the problem keeps happening? Some code somewhere is probably trying to store the database connection in the db or similar.