Same issue here, we have tried all of the above and a few more things. The only workaround is to start a new multiplayer game, invite your friend and then load your latest savegame.
The downside is that we now have to go through this every time we want to play. Would really appreciate it if anyone has a "real" fix for this.
BTW, me and my friend both play on PC via Steam, latest game version, no mods (this is our very first playthrough, we have never installed any mod since buying the game).
Last edited by eperon; 23/09/24 08:02 AM.