long time lurker, signed up to maybe help...
So when I first tried to sign in with HULU on ATV 4, as I typed my username in, the line on screen remained blank like it wasn't "taking" it. I closed the app, deleted it, reinstalled it several time to no avail...
But one of the last times I was trying to enter my username, I heard the "click" sound of the letters as I was entering them. But the line was still blank. Taking a shot, I entered carefully entered my username, not seeing anything and then hit continue or submit, whatever it is and.....it went to the nest screen for the password bit. Typed that in, again without it showing up on screen and hit submit and boom, I was in...
Not sure if this is your problem, this invisible entering of letters but...
Hope this helps.