drush cim creates and immediately deletes field
Okay, there was a second field.field.node.forum.comment_forum.yml in the code of a custom module. It was just a simple setting which sould rewrite the code above but lead into a conflicting import. It looks like the second yml file gets imported first. 
So far deleting the second yml file fixed the problem.

P.S. We will now try to find out why the rewrite doesn't work in the correct order.