Error on save configuration

Problem/Motivation

When trying to add an entity import I get the following error:

Drupal\Core\Config\Schema\SchemaIncompleteException: Entity type 'Drupal\entities_import\Entity\EntitiesImportType' is missing 'config_export' definition in its annotation in Drupal\Core\Config\Entity\ConfigEntityBase->toArray() (line 252 of core\lib\Drupal\Core\Config\Entity\ConfigEntityBase.php).
Drupal\Core\Config\Entity\ConfigEntityStorage->mapToStorageRecord(Object) (Line: 273)
Drupal\Core\Config\Entity\ConfigEntityStorage->doSave('termine', Object) (Line: 524)
Drupal\Core\Entity\EntityStorageBase->save(Object) (Line: 253)
Drupal\Core\Config\Entity\ConfigEntityStorage->save(Object) (Line: 339)
Drupal\Core\Entity\EntityBase->save() (Line: 607)
Drupal\Core\Config\Entity\ConfigEntityBase->save() (Line: 191)
Drupal\entities_import\Form\EntitiesImportTypeForm->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: 592)
Drupal\Core\Form\FormBuilder->processForm('entities_import_type_add_form', Array, Object) (Line: 320)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 73)
Drupal\Core\Controller\FormController->getContentResult(Object, Object) (Line: 39)
Drupal\layout_builder\Controller\LayoutBuilderHtmlEntityFormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 564)
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: 158)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 80)
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: 708)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

The configuration settings look like in screenshot 2022-01-18 212205. The Excel import table is built according to the template as in screenshot 2022-01-18 212435.
I always use a protected private folder (private://...) as Drupal file storage, if this should be important (see screenshot 2022-01-18 213331).