I know I bash Apple products all the time, but I do have to admit that I do own one piece of Apple hardware: the AppleTV (ATV). However, its been hacked so I could run XBMC - a far more superior media center software suite than Apple's built-in stuff (and more open too - it doesn't rely on an iTunes account).
Last night I finally upgraded my ATV to firmware 3.0.1, having waited to make sure all my hacks could still be done. Of course, an upgrade wasn't all that necessary since everything was running quite smoothly except YouTube.
Recently I've been unable to access my YouTube subscriptions, receiving an "Unable to download content" error, even though access to my favorites worked fine. Thinking YouTube changed something, I decided to finally get with the times and update my box. Well, after updating I was disappointed to find that the problem remained.
After some more research (a.k.a. Google searches), I was hinted at the notion that my sharing preferences were wrong on the YouTube side of things. I decided to go to YouTube's Active Sharing account management page and found that I had unchecked "Subscriptions" and left "Favorites" checked (since I wanted to share my favorites on Twitter, but not my subscriptions). This seemed to be consistent with the things that I've noticed - favorites were working on the box but my subscriptions weren't. After checking the both of them and saving, I went back to the ATV and found that it now magically worked as it had before.
So, long story short, if you're receiving an "Error Loading Content" message when you try to access your YouTube favorites or subscriptions, make sure "Subscriptions" and "Favorites" are checked in the Active Sharing section of your YouTube account.