I've had this happen as well, my symptoms were:
I would try to initiate a mulitplayer game or resume a singel player campaign, and it would attempt to load for an inordinate amount of time--ultimately resulting in a "dirty disk" error.
Afterwards, it would immediately give the error upon attempting to load any gametype.
I have moved the files off my box, deleted, re-FTPed, etc--all to no avail.
What has worked for me--Copy a clean version of the came over, and reboot the box. Launch Halo 2, then go to campaign. Start a new game (whichever level), and just wait. It will take a while. You will have cause for wonder and concern, but just be patient. Eventually, it will load. You can then save and close.
Now, in multiplayer, you should be able to connect to a game (via system link, kai, xbc) or initiate your own, only this time the map should load.
Good luck--I know this doesn't help for an explaination... but it may help you work around it.
-B <