I have the latest version (12.6) and when I try to delete a Tab Group and then click OK, I still see the Tab Group. I have Administrator rights on my laptop, so it's not a write permission. I also create a new one and also can't delete it (even if there's no tabs attached to it).
If I go to the /dopusdata/TabGroups, I see still see the Tab Group in this folder and also in Directory Opus. If I delete it directly in the folder, it works, so it seems to be a problem when deleting it from Directory Opus.
If you turn on the Attr / Attributes column, has something set the tab groups files to read-only?
(If so, check the rest of the Opus config folders and your user profile in general. It will cause more problems for other settings/programs as well if that's what has happened.)