I don't get the "None None" near the thumbnail: section of the console output when run, but it may be because all my albums have artwork. I'll have to transfer more from PC when I get a chance and see what happens.

os.rename does have issues when files are on different devices. For mussorgsky both source and dest reside in /home/user/ (.cache/mussorgsky, .cache/media-art/*, and .thumbnails/cropped/).

Just to make sure, please try the attached deb. It won't fix the issue but should print variables (img_path & filename then thumb_path & thumbnail) before attempting os.rename() to see if they're at fault.

Edit: None None does't appear for me with no artwork set. I'll try to go through some more code tomorrow as your also missing alternative-0 lines.

Edit: Remove old test build.