I'm experiencing similar issues with Chrome, some youtube videos will play, others will not. Some youtube videos will play once but then will not play again.
Have you had any luck with resolving this?
I have Flash v18.104.22.168
I just wanted to chime in and say that you're not crazy: I've been having similar issues with Chromium, the FP 11.5 beta, and YouTube too. (You linked to the same video twice, BTW, so I'm not sure if I saw the broken one or not. (It played.))
On the other hand, I can't duplicate the problem today, so maybe it was YouTube's fault.
Moved to the Using Flash Player forums as this is not specific to the Beta.
Am on Mac OS X / Safari.
Had the same problem today after installing Flash Player 11.5.502.136.
I managed to fix the condition by uninstalling Flash Player, installing the latest upgrade of OS X (10.7.5) and Safari (6.0.2) and then reinstalling Flash Player.