Try logging out of iTunes store, then logging back in. If that fails, it's probably time to do
"reset all" If you search on the error here, you will find a couple of other things to try too. Most seem not to work.
After a reset all, you have to log back in (including any WiFi passwords and your iTunes account) and reset any custom user settings. It's a real pain, but doable in less than 10 minutes.
I had the same problem recently. Apple thought it might be either a server problem (possibly one server, or one geographic area, I'm in upstate, NY), or possibly a corrupted firmware download. They also suggested, if it happens again to do a restore over wired Ethernet, in case the last software revision (5.3) which I did over WiFi was corrupted. Apparently it could be a bug in the new Ver. 5.3 too?
It would be interesting to know if tens of us are having this problem, or hundreds, or even thousands of Apple TV users?