I'll make it easy. I'm having the exact same error as this guy here but I can't just call AT&T because they don't give me the support other phones have because mine is a pay as you go phone! WHAT SHOULD I DO?!?! I can't send MMS pictures anymore and it's horrible!
Call 1-800-331-0500 (preferably from another phone) and after you reach the IVR for prepaid accounts say "more options" at the first prompt, then "customer service" at each subsequent menu prompt. The IVR will give up after a few options and connect you with a live representative.
=^..^= There must be a happy medium somewhere between being totally informed and blissfully unaware.