Can't pay online for 4 months now

The last four months I have not been able to pay my bill online. When I select the amount to pay, it takes me to the login screen, I enter my password, and it takes me right back to the same screen. I emailed and called Comcast about this, but the darn thing still hasn't been fixed. Is anyone else unable to pay online? Is there a trick to this? I was able to pay online without issue for months and then suddenly wasn't able to four months ago. Really annoying.