[@italways Member 16 years, 11 months ago]

sorry it is on my local test machine. furthermore, it happens when you try to delete the tag from one topic, you must be a admin user.

The only thing special is that my bbpress is integrated with my wpmu, I logged in using my wpmu admin account (mapped to bbpress admin group already), when I try to delete the tag from one topic, the error occurs, but it is ok when I manage the tags on bbpress backend, I can delete any tag there.