Error after creating a event series

Problem/Motivation

When I create a new event series, I get the following error message, regardless of which event type I use. I also get the error message with the standard type Master:

Warning: Array to string conversion in Drupal\Core\Entity\Sql\SqlContentEntityStorageSchema::castValue() (line 2579 of core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php).
Drupal\Core\Entity\Sql\SqlContentEntityStorageSchema::castValue(Array, Array) (Line: 1068)
Drupal\Core\Entity\Sql\SqlContentEntityStorage->mapToStorageRecord(Object, 'eventseries_field_data') (Line: 1127)
Drupal\Core\Entity\Sql\SqlContentEntityStorage->mapToDataStorageRecord(Object, 'eventseries_field_data') (Line: 1013)
Drupal\Core\Entity\Sql\SqlContentEntityStorage->saveToSharedTables(Object) (Line: 958)
Drupal\Core\Entity\Sql\SqlContentEntityStorage->doSaveFieldItems(Object) (Line: 718)
Drupal\Core\Entity\ContentEntityStorageBase->doSave(NULL, Object) (Line: 520)
Drupal\Core\Entity\EntityStorageBase->save(Object) (Line: 804)
Drupal\Core\Entity\Sql\SqlContentEntityStorage->save(Object) (Line: 339)
Drupal\Core\Entity\EntityBase->save() (Line: 285)
Drupal\Core\Entity\EntityForm->save(Array, Object) (Line: 392)
Drupal\recurring_events\Form\EventSeriesForm->save(Array, Object)
call_user_func_array(Array, Array) (Line: 114)
Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object) (Line: 52)
Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object) (Line: 597)
Drupal\Core\Form\FormBuilder->processForm('eventseries_zeitbereich_add_form', Array, Object) (Line: 325)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 73)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 580)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 169)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 81)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 718)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)