Well, this evening when I got home I tried to fire up YouTube for my son and it hung again. I had tried to connect using the YouTube app on my phone. Same stick splash screen. I restarted the Hopper, after which YouTube would open and play videos, But my phone would not connect, even though it saw the Hopper for casting.
I suspect they still have some bugs.
Several minutes after my post my wife was able to connect from YouTube on her phone.
This morning I tried again from my phone, and same as before YouTube opens but stays at its splash screen. After a restart the app opens on the Hopper but my phone can't connect...stays at connecting for a while then that bar disappears. It seems unlikely theres an issue with my phone since we're both on the latest iOS and my phone works fine with our Chromecast.
I will try opening the app on the Hopper first next time.