Lets be frank: this is an internal server communication issue at Apple that they don't want to fix or acknowledge. This "unable to sign in" or unable to communicate issues are all about purchase validation ONLY. That is where it hangs up. For example, I can see favorites but it won't access my list of purchased items. And yet - the only advice they have is reboot?
The same issue happens when trying to watch Netflix on ATV. It can't connect to the Netflix server but gosh golly gee, I switch to my Roku box and no problem. Unfortunately for Apple, while I'm over there I'm starting to to buy movies and shows from Amazon
Apple doesn't make it's money from the $99 Apple TV sale - it's the iTunes purchases. If we want them to listen we need to interrupt that revenue stream.