When I try to log in to CritiqueBrainz (to edit a review) I can no longer login.

I either get this popup, which I can fill out as often as I like with my MB login details, to no avail (it keeps popping up again):

Eventually, or if I cancel, it goes to a "OAuth authorization error" page, with the text "Error: invalid_client
Unknown client":

There is a long-standing issue where I can't login when using MB beta (CB-464), but this issue now also happens on prod.