So if nothing has changed - why should MP3tag suddenly change its behaviour.
I cannot reproduce what you have described - i can still rename files with Convert>Tag-Filename.
So you would have to keep investigating what still might have changed.
E.g. if this are new files, whether they have not been corrupted.
Or that the file mp3tag.cfg is still ok after you have forcefully closed MP3tag.
This thread lists some of the things that you can do locally to nattow down your problem: