Just found this post now. I have this exact same situation with YouTube locking up on the "loading" message screen. It happen very often for me, and I find it frustrating having to unplug/replug my Humax 1100s just to reset it to fix this issue. but it's not uncommon for me to switch my box on/off several time in a row when this problem occurs.
Besides using Freesat's own youtube app, I've also cast YouTube vids to my Freesat box using the Chrome browser on my Win 10 table, and although it works with this setup too, again I will still get frequent lockups.
Looks like I may have to spend some extra money on a more decent hardware setup, such as Chromecast to solve this issue. I shouldn't have to do this really, as far as I'm concerned this should be working properly on the PVR with the Freesat service.