Thanks Fabian,
one of the best Bugs I've ever seen! :woohoo:
It works in the case that the last file in the loop on the main images path is of image type. That's the $file variabel. Is this the case all files in the thumbs path are in the selection list. I do not know in what order the file list array is filled.
I'm though innocent in this case. I always use the same variabel in such cases. No need for two.
if someone else have problem with seeing and adding thumbnails in the backend.
Untill we have a fix you can check it, if you have the knowledge how, by adding my "debugging" lines. If you see the thumbnails in an array, this is the bug and not the path or the path/file rights.