New threads don't get a thumbnail on the forum page (the list of threads), is that what the problem is? I turned off thumbnails for attachments (so that all attachments inside a thread would always be shown full-sized - you don't have to click a thumbnail to view the larger image), perhaps that made this thing break. Will have a look. Thanks!
Okay, I believe this should be fixed for now. Thumbnails for old posts have been rebuilt. Please tell if something is still broken!