I suspect a large number of these incidents are due to the password field in the login page allowing fewer characters than the field in the sign up page, so the password gets truncated. A couple of help desk meat shields have confirmed that for me, but mostly I think this because it seems to fix itself if I use a shorter password.
How short, you ask? Who tf knows! They sure as shit won’t tell you! Just spend the next 20 minutes trying shit til it works, because you have nothing better to do with your time!