I would try the login information set as System using the format for the username Domain\User (or ComputerName\Username). Usually that will fix most of your login error. Please post back if this does not work.
I have a 5135 MFP that network scanning does not work correctly. If I choose to authenticate using "System" and enter the username and password, it always fails to scan with a Login Failure. If I set it up for Prompt at User Interface and enter the same username and password it scans just fine. ANy ideas?