It's not your username/password.
Similar threads exist. I'm unable to watch ABC, NBC (which is funny given guess who owns them..) or FOX because they all say I don't subscribe yet these are basic local channels.
You only seem to be able to watch old stuff that everyone can watch but most the new stuff is restricted, except for ABC that flat out restricts everything.
Anyway http://forums.comcast.com/t5/XfinityTV-Website/Says-I-need-a-subscription-to-watch/td-p/1967841 has the run on of no one helping...