I wrestled with this problem for about a week calling and chatting with both HBO and ATT-Uverse (my TV provider). Each blamed the other (what's new?). Watching HBO GO on the ATT-Uverse website on my computer didn't work either as it had before. The error message I got was "Invalid affiliate". Now I was really dead in the water, no HBO GO on ATV or on the computer. I called ATT again and after about half an hour of give n' take the agent asked if I had tried viewing HBO GO on the HBO website (I hadn't). I went to the HBO website and registered which allowed me access to HBO GO and aparently cleared up the issue with ATV. Everything has been good for the past 3 days.
Here is what I did: Went to hbo.com and registered (it''s free). I identified my TV provider (ATT-Uverse) to complete registration. Once registered and signed in, clicked on the HBO GO link (top right corner). The HBO GO website is split in 2 (ignore the right side). I clicked on the Visit HBOGO.com link and a new web page opened. I clicked the Sign In button (top right) and a mini-window opened asking to select my TV provider, which I did. I was then directed to the HBO/ATT sign in page and signed in to my ATT-Uverse account. And now I'm able to access HBO GO on ATV.