I think I have been hit by the same bug (Discourse version 2.7.10, upgraded today).
I was granted admin rights, and enabled 2fa. Later, I created a second user for myself from a different email to be used solely for admin, and granted that one admin rights.
Then I removed admin rights from my original user, and deleted all 2fa methods. Now my original user gets the error behaviour mentioned by the OP.
I don’t have console access to this server. The site is not enforcing 2fa for admins.
Would somebody please detail what exactly needs to be done at the console to unstick this problem.
BTW the reasons I have done this is that I want to be subject to normal permissions and notifications (or not) for private groups on the site via my normal user, perhaps with mailing list mode active. I don’t need the extra friction of 2fa as an ordinary user.
Of course the admin user can see all, so it is on trust that they don’t snoop unneccesarily. That user can leave all notifications switched off, and not enable mailing list mode.