Every time I run XBMC (launched via Classic TV shortcut), I can't stream anything to the YouTube app. When I hit cast in my browser and choose FireTV, the YouTube app just exits back to the AFTV home screen. Only restarting Fire TV fixes this issue.
I see the same thing without Classic TV in the mix. I assumed it was just a problem with the YouTube app.
still a problem after 6 months
I had the same problem but it also effected Netflix, iPlayer and other Android apps on my AFTV.
I have disabled LibStageFright in Settings>Video>Acceleration and it seems to have solved the problem.
That's not a good solution, because LibStageFright decoding is excellent and provides smoother playback of 1080p files.