I had the same problem (twice now). After enduring 45 minutes on the phone with their level 1 and level 2 "support", they ended up "re-provisioning" my modem, which I think was essentially rebooting it. After that the login page came up fine. But again, this is the second time it has happened to me, and although they assure me they fixed it "permanently" this time, we'll see happens the next time I log in to pay my bill.
You would think they would ensure access so that I can at least pay my bill.
I have the same problem! It doesn't matter what web browser, any attempt to log in to my Xfinity account on a desktop browser fails with something like:
> Firefox can’t establish a connection to the server at oauth.xfinity.com.
I had to tether to my phone's connection to get connected just to post this response!