Not sure what else to think of, accept make sure that you are using your nowtv account sign in credentials on the ATV rather than your iTunes account details on the NowTV App.
Just signed out and back into my ATV Gen 3 box without no problems.
Hopefully, somebody on here will have the answer and come to your rescue.
When you say you are getting Authentication Errors on the Forum, is that when attempting to write or post a message on here ?
If so, they are two different issues where the Auth error on the forum shouldn't cause the sign in error on your ATV.
Hopefully other ATV owners will offer some suggestions on what things to try.
I am having the same auth error on an Apple TV. The above ‘work around’ did not work for me either.
When I restart the ATV, I can briefly access the app but if I try to watch content then it says that the site is currently unavailable. This is all despite my being able to watch on my phone and laptop.