When using the function
Tools>Options>Cleanup database
it is possible to close the "Options" dialogue without terminating the cleaning up - the progress dialogue for the cleanup process stays open.
If you select any (different) file in the files list it leads immediately to the display of a message box that an exception error occured and the application is in an undefined state.
The cleaning up continues, though.
While the "Options" dialogue is still open, it is not possible to select any other file.
Found in 2.87d
I've fixed this with the current Development Build Mp3tag v2.87e. It's still possible to close the dialog (at least on some systems) and the editing should not be possible anymore.