On Halo 2 Anniversary, I do not appear to be able to load any missions from the Campaign in Co-Op, with the exception of Cairo Station. Missions load normally when played in sequence, but if I were to choose Outskirts to start on, for example, it isn’t possible.
Bug appears on all difficulties and appears to be independent of skulls time/score display choices.
Co-Op party is over Xbox Live both connecting using BT in the UK.
Have you tried resetting your internet and xbox at the same time, both you and your friend.
I had this problem and it’s so annoying. Somehow we fixed it by going through these steps. First, have both of you load the level from the start in solo campaign. Then immediately save and quit after the cutscene. Next have your friend send you an invite to their party and press resume. If that doesn’t work, then you could try loading the level with classic graphics, and also make sure you’ve both done The Armoury. Hope that helps