I've been a Comcast customer for several years between several different appartments, most recently I've been a customer at my house for 2 years. I use my own Arris modem. One week ago I swapped my SB6121 for a SB6141, everyday since I've needed to reactivate the equipment with Comcast. Seems to be exactly what is described in http://forums.xfinity.com/t5/Your-Home-Network/Upstream-port-80-Blocked-on-modem-or-network/m-p/2372....
The "solution" is to PM the modem MAC address to get something fixed on Comcast's side. However I can't seem to PM ComcastJohnN. This is my first visit to the Xfinity forums, but after reading this it would appear I need to make a post first? So this is my attempt to get support for this issue.
Activation can be done by directly connecting your PC to the modem via Ethernet.
I can't say that I've seen where a modem / router won't allow port 80 http traffic and allow 443 https. There's plenty of posts though where if the system picks up the MAC of your device and only allows ipv6 traffic to work due to the MAC in the billing system either being wrong or designated as a DOCSIS device instead of CPE. Maybe one of the CC reps will pick this up that are familiar with the issue if re-registering online doesn't fix the issue.