Check out the hints here
http://discussions.apple.com/thread.jspa?threadID=1980815&tstart=0
This part fixed my problem:
4. Okay. So you're logged in. You can search. You can find stuff. You can view history, etc. But you still can't see your favourites and subscriptions? It gives you an error? Trust me. This problem was wracking my brain for months. This is how you fix it. Open up your desktop web browser (Safari, Firefox, etc) and log into YouTube.com using your YouTube account. Once logged in, in the top right next to your username click on 'Account'. Once there, click on the 'Privacy' link to edit your privacy settings. Now here is where the weirdness comes in. Under the 'Recent Activity', uncheck all the options and hit save. Then go back and check off, at least, both 'Favorite a video' and 'Subscribe to a channel'. Hit save and exit your browser. Restart your Apple TV for good measure and log into YouTube again on your Apple TV. You should now be able to see your favourites and subscriptions. If at anytime you find this method fails, load up the browser again and repeat this process. It should 'reset' whatever the
** the Apple TV does to screw it up in the first place.
It appears the Apple TV checks your public settings rather than actually using your account. Your account is only used for verification purposes, otherwise it is in no way really tied to your account. This is proved when resolving the last step as well as not being able to view your browser-based history or recent searches. The Apple TV YouTube interface is simply a front end much like the app found on the iPhone/iPod Touch.