I’m unable to delete groups held in the main inbox. I can move them to the trash and empty the trash, but they come back after I reopen DT.
I verified and repaired, in case that would help. I also deleted the folder from the Library/Application Support folder. Neither removed the groups when I reopened DT.
I concur with @chrillek and I don’t think the 5-year old post you referred to is fixing a problem with a group that does not delete. @bluefrog can comment on possibly what damage you did to DEVONthink system and the way forward.