I have been experiencing the same thing. Thought it was just me. Sometimes it works fine other times I have to reset. The other weird thing that happens to me is that after it doesn't work and I have to reset, go back to Netflix it is working, but I have to log back in to Netflix.
There was another thread about this subject and the cure was to reset to factory defaults. Then, of course, you have to remember all the things you changed after install.( 0n 52 remote hit home 3 times , select tools on diagnostic screen and then select reset to factory defaults.
My YouTube worked fine in the week since installing the H3. Today it gave me the black screen. Tried the reset above but still don't work. In fact, it forces a reboot when trying to go back to live tv.
I did follow the instructions in post 7 -- several times. At least twice my H3 became unresponsive for maybe 30 seconds, then it rebooted/restarted on its own. YouTube was not fixed after doing ANY of the resets.
I did some more digging online and saw that you can also cast YouTube from a portable device (in my case an iPhone 6) directly to the H3. Casting it apparently hoses the H3 too and not just the built-in app. The red YouTube logo appears then the screen goes black after a few seconds. Hitting the menu button eventually gets you back to the menu screen but then nothing happens until it completely reboots. Not good! I hope they fix this as it would be much more convenient than having to switch inputs to my Roku or Apple TV.
My Netflix works fine but my YouTube goes to black screen after the initial Splash screen. I just got the H3 installed yesterday and thought it needed all it's updates before working. But I see here it's a common problem.
Count me in as one having YouTube issues, too. New Hopper 3 installed 12/2. YT worked fine that day. 3 days later, I get the red splash screen for a few seconds, then black, and the Hopper 3 reboots itself about 3 times. Tried resetting YouTube to defaults but no luck here. After the 3rd reboot and Starting Up splash screen I get a small window in bottom right corner saying "Please Wait, Refresh in Progress, 9999" and then it eventually goes to live TV or turns off and I can power it up and watch TV.
Also having an issue with Pandora. It logs me in and starts playing a station but no audio and plays maybe 3-4 seconds of a song then skips ahead, and keeps doing this, even skips passed commercials like this. Looks like some software fixes are needed for Apps.