I've had the same problem recently and spent 2 hours over 2 sessions with support trying to get it fixed in vain. In the process the last service rep deactivated my HBO entirely so I couldn't watch even using the Comcast box. I finally stopped into my local Comcast office and an agent there said he'd been hearing from a number of people that there is some problem between Comcast and HBO systems completing HBO Go activations and that Comcast and HBO were working on the problem. Why this info wasn't available to the reps I'd worked with to save me all that time I don't know. Very poor internal communication and customer experience.
I'm in this same situation as of yesterday. Can't log into HBO Go or Showtime Anytime. Nothing changed with my account.