update: I appear to have solved the problem by opting out of the Steam client beta thing. Something about the newest beta update made Workshop services and server hosting not work properly in the game.

I also am unable to host servers (non-dedicated) with the DLC enabled. Re-downloading it now, but with the DLC enabled, can't host, with it disabled, I can host just fine. I suspect these errors are connected somehow.

I'd also like to add that this error happens no matter what the contents of the mission are. Blank slate and fully made scenario give the same error.