I'm having a similar problem, i also stream videos from tversity to the 360 with no problems.
but i have recenty played a few pre-release games! sooo i have stayed off live, but when i tried to play a video the other day that i know works perfectly well, it comes up with the missing codec warning and refuses to play.
But as soon as i log into live it plays perfectly even though it doesn't download any update as far as i can see, then i log out and bang codec error again!!
So it seems some people have to be logged into live to be able to play certain videos 
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