As said above, be sure to have OWB 1,23 and latest scripts at http://fabportnawak.free.fr/owb/scripts/ (youtube.js and youtube_fullscreen_fix.js, only those two are really needed for youtube). And as said above, be sure to ADD/ENABLE them in scripts manager window.

I just tested a few seconds ago, and all the videos i stumbled on worked, either in normal HTML5 mode, or in Flash-replacement mode.

And finally, as said above again, it's not morphos or the browser's fault if Youtube can sometimes break. They only have a very small fraction of their videos available as HTML5 mode, and for the rest you need hacks to replace Flash, which can obviously break each time they change something in their page.