It's a known issue,still not fixed: http://forums.xfinity.com/t5/Customer-Service/Can-t-manage-your-account-online-Getting-an-error/m-p/...
Hi tdzilla76 -- We can help you manager your plan here due to the ongoing issue with online plan management. What changes are you looking to make?
tdzilla76 -- I'll need to review your account to answer that question. Please send me a PM with your name, full service address, and either phone number or account number for authentication. I'll review pricing in your area for upgrading.
Click my name (ComcastZach) and click Private Message Me.