i had this problem and solved it by simply rebooting my imac
heres what happened to me...
i updated itunes to v11, watched a few movies and then tried to add some tv shows
i couldn't find them anywhere, i restarted itunes trying to fix this problem
this is when i got the same error as you, i knew something wasn't right as i had just been watching videos but now NOTHING would play
all i had done was exit itunes and open it again!!!!
i simply rebooted my imac and videos started playing in itunes again with no quicktime error
i hope this works for you