Hey, I can confirm the problem. The game Crashes to Desktop when loading a coop campaign savegame.
I followed the guide, created the shortcut with the /init init_coop.lua prefix, successfully started the coop campaign game as Skirmish, saved and tried to load via Campaign menu again (tried with Artifact mission).
So the problem still occurs unfortunately. Would be awesome if it could be fixed.
Btw.: has their ever been attempts or ideas to introduce saving/loading online matches? Normally I would think implementing something like that woul be impossible, but seeing what you managed to already do with the game makes me wonder.