I am getting the same message from Avast. I am currently playing on-demand video from Comcast/Xfinity without problem, but the error message does keep popping up.
I get this popup continuously since the latest update: "AVG has blocked access to https://analytics.xcal.tv/ because the server certificate has been revoked."
I'm having the exact same issue. I'm guessing no one has figured out the problem yet. I think it's slowing down my system, too because it serious is continuously popping up. Urgh.
Just FYI, I went to the AVG site and saw that we can just add that link to the exceptions area until Comcast can get the certificate fixed. I did it and the pop up is gone now.