How to replicate this bug:
1] Load some files to Mp3tag
2] Start writing any filtering expression; even as simple as ab
3] Select some of the filtered results
4] Use right click menu and choose Move
5] Relocate those few files to wherever you like
6] Now move the mouse pointer to the box of the Filter and left click in it
7] Press SHIFT + DEL in order to delete the filtering expression
What happens? You should get an pop-up window An unexpected error occured. Would you like to save a diagnostic file?
This happens in version 3.16a x64 on Windows 10 Enterprise 20H2 19042.746 x64
[And by the way: there is a typo in that message - as the proper spelling is occurred and not the used occured]