I'm going to resurrect this thread because I've been having the same problem for a long time, as have many others:
https://forums.halowaypoint.com/yaf_postsm2822624_Halo-Anniversary-System-Link-Broken.aspx
http://forums.xbox.com/xbox_forums/xbox_support/f/8/t/355412.aspx
http://www.neoseeker.com/forums/54478/t1774629-halo-anniversary-system-link-co-op/
http://www.gamefaqs.com/boards/632871-/61108693