I was having the same problem, then I discovered a work-around.
Try this:
Try to sign in on HBOGo as normal. When you are redirected, sign in with your account on the Xfinity website. Keep the Xfinity page open and open a new tab. Using the new tab, sign in to HBOGo as normal.
It would seem that as long as your browser is actively signed into the Xfinity webpage in one tab, HBOGo will recognize that you are already signed in. Hope that works for you.