Did you ever figure out what it was? I have the same problem. Used my apple tv for 11 months now it won't allow log in no matter what we do. Log in works on pc but not apple tv. Tried resetting wifi connection and others but no go.
I'm also having the same problem. Everything was working fine but now my ATV will not recognize my itunes account. Ive tried all sorts of suggestions but nothing has worked so far.......so there it sits a useless little black brick!
It's really frustrating, and forwarding ports is not something I should have to do. But let's face it - iTunes is losing money because I'm not buying their tv shows and movies! You would think Apple would fix this.
djedefre - you were kidding, right? You don't really think that's straightforward.