I’m getting the above error on Plex mobile (Android).
Error has now changed to just: User could not be authenticated
I’ve checked a suggestion from another post:
Are you using spaces in your password? Or one particularly rare special character at the beginning or end of your password? I have neither. user-could-not-be-authenticated/722849
I see that you had a bunch of failed sign in attempts. We do block an IP from signing in after this. These blocks last about an hour and go away on their own. But if you fail singing in again or try during this hour, it will reset. Just wait a bit and try again. Make sure there are no other device on the same IP trying to sign in with the bad info either.