I'm sorry to say that Comcast has been very tight lipped about what these 900|etc error messages are, in some cases a Comcast person has acknowledged that its a problem on the back end of their system and that they know about it and that it's being worked on. In any number of instances when I've gotten it (not sure if it's the same 900|xyz error as you're getting), it's usually fixed in a day or so.
Your best bet, temporarily is to go directly to the networks own website and watch from there.
- Norm