This is kind of a necro revive, but I'm having the same issue as OP however it only ever happens when we try to play Maelstrom mod R5.
We have no problems hosting saved games with the original game nor with other mods, but when we play Maelstrom mod we get this error every time we try to host a saved game. We always save before we disconnect so that isn't what is causing this for us. We've also loaded autosaves and we get the same error.
Our checksums are the same, and we can host a new game no problem.
I can send him the save file for a temporary fix and see if that works, but I'd rather fix this problem from happening again.