I saw your post yesterday and thought the problem was really odd, until it happened to my room mate and I as well playing co-op over live. The game froze up during the cutscene to data hive, but we let it site for about 45 seconds on a black screen and finally it let us into the mission. Once we finished the mission the game booted us and said that it lost connection. We went back in and were able to get into the city. The cutscene acted really weird here as well and we were able to get it started by skipping the cut scene (again, it was just a black screen). We got to Coastal Highway and got booted again, this time we had to do the whole city part over again and skip the cutscene for Coastal Highway but we were able to save at this point.
I think something may be wrong with the game, maybe it has something to do with the update a few days ago.
I think I have figured out the problem. The game tends to freak out in this section if the game is installed on the hosts hard drive. Did your host have the game installed? Try deleting the install and try it again. This worked for my friend and I.