It's a known bug. As for me it's regarding to custom factions with own graphics. Reason still unkown. 

Following solutions partly helped:

- block Steam-cloud from synchronisation (Steam - preferences - cloud - uncheck)

- delete all earlier multiplayer-savegames (you may also delete singleplayer as well)

@Raynman

Thx the question and 'yes'. Support Ticket ID: JYJ-143-25157

Yes.  problem  is still there. it completely spoils the game. impossible to start MP. after several attempts to load MP - I just gave up.

another issue - very frequent crashes at every stage of the game.  

1. if you lucky to pass the point off "loading custom ship data" - the chances are: still you can crash at the very last moment. 

2. crashing happen often as you try to  modify your ships in design tab.

 the game is still pretty much RAW not in terms of its' gameplay, but in terms of it's stability.

It's a shame, I have always enjoyed GalCiv even though I always had a problem with 2 crashing in long games. What I always thought would make this game better is for it to be multiplayer. Now I have not had even one clash playing 3, even in games that have gone over a 1000 turns, it has run great. I actually bought 3 for a friend in another state so we could game it together, and this problem has occurred. I hope it gets corrected soon.