Hmm, really weird this. The last time I activated thumbnails with unregistered users cannot view forums too, I rebuild the thumbnails but didn't work.
Now, I just tried to activate the thumbnails again, and it suddenly started working.
Really weird, but it works now!
Thanks for the respond!
[EDIT]
It randomly stopped working again. I uploaded an images to a thread and the thumbnail that should be appearing, is not there. I can click on the space there, leading me to the actual image. But the Thumbnail is gone. Same for all the thumbnails on the other pages.