There's a bug in Mp3tag 2.35m that takes focus away from the main dialog after resizing the "List of search results" and the "Adjust tag-information" dialog which is called from the previous. When you resize either window (you may have to try it a couple times to see it), you will notice draw errors in the dialog. You can close the dialog but it's like it's still open and you cannot focus on the main dialog to close Mp3tag. You must kill the application.
This is easily reproducible and I've done it many times just to make sure. I haven't tested this in any other version though, only in this latest beta.
Thanks again for this awesome program!