Youtube bug in Safari with YouTube5 extension

Hi,

Some days ago I've noticed that Youtube movies don't play correctly in Safari. I don't have Flash Player installed but I've been using YouTube5 extension for Safari for a long time and everything worked fine. I've recorded a video of my problem: http://www.youtube.com/watch?v=xOk5sJAlSRU . After a while webpage asks me to install Flash Player, but at the beginig it looks like loading movie correctly.. Can somebody help me with that? I've tried resetting Safari (in Safari menu), turning on/off Yotube HTML5 and nothing helps. I must add that embeded videos work OK, extension loads its player.

My specs: OS X 10.8.4 , Safari 6.0.5 (8536.30.1), extension YouTube5 2.6.1.

Thanks,

Kuba