The fortress borders mod used in this modpack is made by me So it is not the original. The original is indeed not for multiplayer. I will take a closer look at your lua log and what is causing the issue with fortress borders... A short look at line 813 in my fortress_borders.lua shows, that "iFortOwner" seems to be nil... but I think it can't be nil... so at the moment I have no clue, why it happen... Are you able to tell me what you did with the outposts? Did an allied unit stand on one of your outposts or simular? Maybe you can check lua.log during the game, and tell me the situation when the error accures? (it is during processing a players turn). So tell me which units stands on whos outpost.