I am having an issue in the same location. While playing through the campaign, immediately after jumping through the portal at the end of Requiem, I received a "disc unreadable" error.
Upon restarting and attempting to continue the campaign from that point, the game takes a very long time to prepare the game. I am able to start the mission, but a half second into the loading splash screen the game freezes and gives me another disc read error.
I have examined the disc, and there are no scratches or foreign objects/substances on it. The game only left the case to be transferred to the console, and there was no opportunity for it to be damaged.
Please advise.