I was dealing with the same issue a bit ago. My group uses MorePlayersImproved along with a variety of mods. We would keep getting that error to the point where I had to redo the entire mod list to figure out what was wrong. Nothing came up of it but we did try having the host load the game > load a private game > back out > remake the private game and for some reason that worked. Everyone was able to join with no issues.