In addition to other reported issues I've noticed that
Rebuild thumbnails does only work on covers - artists (... _photo directory) won't get updated / rebuild; since XMM is trying to find artist thumbs during browsing this is not just a missing feature but a bug - in my opinion.
When rebuilding thumbs with the supplied function it seems like XMM is doing this record by record / image by image; that's painfully slow and I wonder if it wouldn't be much faster to generate the cover list and then process the images in batchmode and in background? BTW Irfan Views way to occupy the mouse cursor to show its busy state is annoying because it ignores the window state (it's running in the background but still sets the cursor to busy continously).
(Not a bug): would it be much of an effort (once programming XMM resumes) to implement a configuration setting which deactivates thumbnails? I mean, not just by manually clearing the respective folder but by also supressing related search operations (looking for thumb files) during browsing?