Maybe this will explain, if not help. There's an ongoing known issue: http://forums.xfinity.com/t5/Customer-Service/Can-t-manage-your-account-online-Getting-an-error/m-p/...
Sorry for the inconvenience. As mentioned in the link posted above, this is a known issue that we are actively working to get resolved. If you would like assistance discussing what promotions are available in your area, please feel free to reach out to me in a private message and I'd be more than happy to assist.
To send a private message click on my name "ComcastChe", then click private message me. In the body of this message please verify your first and last name, the phone number, street address including city state and zip code, and the full account number or phone number associated with your services.