i had this problem too (on both apple tv 160gb and apple tv2) and logging out and logging back in again solved it. sorry i can't be more help but i thought i'd say this in case it helps others with the same problem.
There is a fix for this. Search the forum. You basically have to go to youtube.com with your desktop browser and adjust your "Activity Sharing" under account settings to include "Subscribe to a channel" and "favorite a video".
There is a fix for this. Search the forum. You basically have to go to youtube.com with your desktop browser and adjust your "Activity Sharing" under account settings to include "Subscribe to a channel" and "favorite a video".