When I play a video on my phone's speakers it works fine. And if I start a video and then switch to the speaker via airplay the video will continue playing.
But if the speaker is already connected and I try to play a new video then it won't load, saying "Video playback error tap to retry." No amount of retrying will work.
It seems like Youtube is prohibiting videos being played on certain audio sources? Or certain kinds of videos (music)?