The main cause for this is trying to access your email account with the wrong username / password details. Remember to use the full email address (firstname.lastname@example.org) as your username when logging in. If all else fails, simply remove and recreate the account with your mail control panel.
If none of the above helps, then most likely the mailbox in question has received a corrupted message. In such an event, contact support and tell them to clear that box out. Note, however, that this usually results in most (or all) of that particular mailbox's contents being erased.