MP not loading means you fucked up your shared.map. Campaign could be the same reason.
https://dl.glitchyscripts.com/halo2/maps/
Redownload the resource maps. Those being shared.map, singleplayer_shared.map, and mainmenu.map. You should have 41 maps in total in your vanilla H2 install.
There is no reason the coop menu itself would cause you to not be able to load anything or renaming a map for that matter unless you edited the shared maps.