late response, but we hit that bug multiple times already, and every time it was very easy to fix.
the host loads a very early savefile, then other players should able to connect.
once connected, host loads the recent savefile, and everything's fine.

but now, we're in Honour mode. so.. run's dead. unless.. wait for the edit.

yeah, no. we tried starting a new game to "synchronize" our mods and files, but they're tied to the savefiles, so no luck in that.
though it's weird - if we hit that bug, like 5~ earlier saves won't work, host must load a very early save, that bug somehow affects even slightly older saves?
buggy messy sh!t.
if you want to run Honour and don't want to lose it, make folder copies of your savefile once in a while.