I'm having the exact same issue. Happened since the new patch. Wasnt ever an issue before. I tried un installing and re-installing, loading earlier saves with just myself and then with my partner...doesnt matter.
Single player works loading from miltiplayer saves.
I wonder if its from loading a multiplayer save as single player, then progressing, saving and then trying to go back to multiplayer after it saves as a single player file...
But regardless, thenafter you cant load a single save file from multiplayer. You just crash to xbox home.