Since the patch you have to click "next" in the mod menu once you've selected the mods you want to play with, not "back"
full path is : main menu -> mods -> next -> single player -> custom game
I knew I was missing something obvious! I guess that, since this is the only mod I use, I didn't notice my general issue. Even though this was apparently unrelated to the mod, and I'm a total idiot, thanks a lot for the help!