Have run into this issue multiple times and have had more than a few 1+ hour long phone calls with Comcast while the rep struggles to solve it and then it only happens again a few months later.
Posting here in case others have found a permanent fix. The problem:
1. Hbo Go kicks us out
2. Click "Sign In"
3. Prompted with activation code
4. Go to hbogo.com/activate and enter code correctly
5. Instead of activating, it just goes to the generic Xfinity account home page
Anyone else have this issue? Working on escalating now to Tier 3 support at Comcast, as it's pretty infuriating to pay for HBO Go and have this happen constantly.
The solution is this: start a call to Comcast TV support (not account support) and ask the tech to do a "UDB refresh".
You are not the only one with this problem! It has been documented by many people over many years. I just ran into it and after many, many calls to tech support explaining the problem and being transferred over and over and disconnected multiple times I finally got it resolved.
They may ask you a bunch of other questions about what you are doing (every tech I spoke to seemed unaware of what HBO GO is and/or how it is activated) but be insistent and use the exact phrase "UDB refresh". After talking to at least 10-15 support reps over the course of 2 days that was what solved it!
BTW, I got the solution from this thread: https://forums.xfinity.com/t5/Stream-TV-App/HBO-GO-Activation-Not-Working/td-p/3083002
Good luck and if you run across this problem in other threads please post this solution there! Too many people have had this problem for too long and Comcast tech support apparently still has no clue how to fix it.