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The official Comcast people have stated that 900|bla bla errors are network anomoly's; could be on their end but it could also be on your end; the first thing to do is powercycle your modem and or router.
They have been 'working' on this problem since 2009. It is NOT a 'problem' that they want to solve. It allows them to reduce how much they are paying for a movie without outright denying that movie to a whole LOT of people.
It's all about the money.