Running 11.10, with up-to-date Flash (18.104.22.168, which the Flash site agrees is the up-to-date version). Every Youtube video reports a blank screen with the words, "Invalid parameters":
I've tried fiddling with the player "hardware acceleration" setting, and that has no effect. Similarly, I've clicked and un-clicked and re-clicked the "storage" options in the global settings thing, also to no effect.
I haven't seen this in any google searches (though there are other problems described involved the "Invalid parameters" message) which makes me think it's a recent phenomenon.
edit — hmm, fails in Firefox but works in Chrome ...