Any Username And Password Error
The main cause for this is trying to access the e-mail account with the wrong user / password. If this is NOT your primary e-mail account remember to use the full email address (email@example.com) as your user name for logging in. If all else fails, simply remove, and then recreate the account with your mail control panel.
If none of this helps, then most likely the pop3 box in question has a received a corrupted message. Contact support and tell them to clear that box out. Usually this results in most or all of that particular mailbox's contents being erased.