WhatIsSol: Some suggestions to try:
1. GOG Galaxy is required for multiplayer (no direct LAN support, no cross platform support with Steam)
2. Make certain you all have the same game version
3. Make certain you all have the same mods enabled and the same load order, personally I would try it with no mods at first.
4. Launch the game from the GOG Galaxy client
Give these a shot and see if it corrects for you.
While not truly the solution I figured it out - just forgot about the question.
The game has multiplayer DRM - unique keys for each version of the game that requires Galaxy to verify. As such they also removed LAN support, so if you want multiplayer - even in your own home with your own family - you need a copy for each member and a sever verification.
With GOG being a "DRM Free" company this leaves a very rotten taste in my mouth.
