And just install latest available scripts at http://fabportnawak.free.fr/owb/scripts/ (youtube.js and youtube_fullscreen_fix.js). Or use Grunch to update them.
In latest youtube.js, I added support for the new youtube links that have an encrypted signature (more and more widely used), so they should hopefully all play (until youtube breaks it again).
You can also have a look at this youtube_centre userscript BootWb mentioned, it works nicely (but you'll have to configure it to use HTML5 and so on).
