Can't create any new article after an upgrade to Drupal 9.x

I have upgraded from Drupal 8.9.2, which had no errors, to Drupal 9.0.2 - everything ran smoothly. No errors. But when trying to create a standard Article, the core library gives me the following error:

The website encountered an unexpected error. Please try again later. Error: Call to a member function getAccountName() on null in Drupal\node\NodeForm->form() (line 155 of core/modules/node/src/NodeForm.php). quote Drupal\node\NodeForm->form(Array, Object) (Line: 106) Drupal\Core\Entity\EntityForm->buildForm(Array, Object) call_user_func_array(Array, Array) (Line: 532) Drupal\Core\Form\FormBuilder->retrieveForm('node_article_form', Object) (Line: 278) 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: 573) 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: 57) Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47) 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: 47) Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52) Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23) Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 705) Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

What is exactly causing it? My modules are: Schema.org 8x-16 (the latest), Metatag and Google Analytics - are the latest. Standard Bartik theme.