My recent experience with GC3 multiplayer suggests that if you enable certain combinations of mods, loading a multiplayer session won't work (see https://forums.galciv3.com/488497 ).
1. enable Crusade and Intrigue. Success.
2. enable Crusade, Intrigue, and several other mods (but not Retribution). Failure.
3. remove everything. Success.
4. enable Crusade, Intrigue, and Retribution only. Failure.
5. enable Retribution and Intrigue only. Success.
Before each test, we cleaned My Games\gc3crusade or My Games\Galciv3, whichever was applicable. We used a brand new game for each test.
My friend has Galciv3 installed under c:\program files (mine is under c:\gog\galaxy\...). He's happy to try again under a different directory if you think that'll help. We're otherwise at the end of our ropes and patience.
Any suggestions would be most appreciated.