You're referencing a Drupal 8 block via block reference but after saving the entity reference input field says "restricted access" (German: "Eingeschränkter Zugriff")? That's simple to solve... and "works as designed".
This post saved you a lot of time and headache?
Then I'd be very happy about a delicious coffee! Or a helpful book from my wish list.
Or are you looking for professional web software development?
Our experienced team at webks GmbH consults, develops & implements individual, innovative web, eCommerce, Drupal CMS & mobile applications.