I am currently working on a theme for a client and now just got into a problem: When I want to set an image (also existing/already uploaded images don't work) the editor just not accept it and don't set it as the post thumbnail for the post.
Any help would be appreciated.
EDIT: Here is an screenshot for the case that somebody didn't know, what I am talking about:
It's like this before and after the "change". So what's going on there?
Normal file upload is working.