I'm having the same problem. I've had a free account for ages and hooked my xbox up via ether net for the first time to try out a kinect. I ran all updates and when I tried to sign in I couldn't.
I then came online on my PC to verify all my details were OK. Every time I try and access certain parts of my account online, I get the 'Uh oh... 'message! This is extremely frustrating! I can't use xbox online from my xbox or even properly from this site. I tried Chrome, Safari and Firefox, cleared cache, cookies, etc. and it's still the same