Friend and I try to play multiplayer. Game starts fine. However, the game will crash for us both after around turn 15, sometimes longer sometimes quicker. What causes this? We've tried reinstalling, even reinstalling the steam client. Game is cleared for firewall.

I am running Win11 Home, 12th gen processor, 32G, RTX3080
Not sure what my friend is running but probably not far off from my system.

The only other anomaly is that my friend complains that using VOIP through steam, cuts out frequently (it does this on CivIV as well but not on any other games we play). My system VOIP is always clear.