Half of YouTube not working in Safari

since 5.1.4 update on lion. else noticed this? approximately half of videos on youtube refusing play / showing black box playback buttons in wrong place. latest version of flash, working fine in chrome.

ideas might have caused this?

( example - video me broken, hope isn't against rules http://www.youtube.com/watch?v=5uelk6vmbtm )

 

that video works fine me, have extensions or plugins might conflicting?

 

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