Drupal 8 Entity reference blocks: restricted access (Eingeschränkter Zugriff)

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".
The reason is simple: Drupal correctly checks for access permissions on the block. But perhaps you've put the block into the "disabled" block region, because you don't want to output it normally.

So how to fix it:

1. Create a new region in your theme, which you do "not" output in you templates. Name it as you want, for example "Usable (hidden)".
2. Move the blocks to that region (from disabled).

Now you can reference the blocks without restricted access message.

Please leave a comment if that helped you.