I had some files with erroneous extended tags - two extra fields named "ARTIST " and "ARTIST - ", containing the filename. When using the extended tags dialog for all the files with those tags, it is impossible to delete those tags. When I try to blank the field with the extra space, the proper artist field is selected for blanking. When trying it with the "ARTIST - " one, an extra, fourth artist-field suddenly jumps up in the box. When selecting the files one by one everything works as expected. It's reproducible, just tag two files as described.
And now - how do I get rid of those two bad artist values I just filled in to test this, that now appear on the drop-down list when I want to add extended fields...? It's in the registry somewhere?