I just started having the same problem. I've had Halo for almost a year now, and when I first played it I could play all the levels just fine. I've played it on and off only on matchmaking. I recently decided to go through all the levels again to get the skulls. All the levels played fine, except when I got to the ark, it would load from 1%, to 2%, all the way up to 5%, then load to 20%, and then back to 0%. It then would say players failed to load content. I know that my xbox has scratched the disk, and xboxes are known for that. But everything else played fine, except for the ark. I even played co-op on cortana with a friend, and it loaded and ran smoothly. So, for me, it's just the ark...