I finally spoke with level II customer service tech support. It was confirmed that there is an issue between HBO Go - Apple TV and Xfinity. Xfinity is aware of the issue and is working on solving it, however they do not have a date when this issue (technical, contract...) will be solved.
Why Xfinity doesn't tell their Level I customer service reps this information is beyond me! Just hearing the honest information that there is indeed a problem was helpful. I will no longer try to activate HBO Go, be sent to the Xfinity page, then call and be told to contact HBO or Apple.
I was told by more than one Level I customer service rep that the problem was HBO Go's or Apple TV's. Please share the issue with your Lelvel I customer service reps, Xfinity. This was a frustrating experience,
Thought I would share what I found out. The problem is in no way "solved." But information is helpful.