It seems that this process fixes the MP crash for me, on start up at least
1. Uninstalling the game
2. Deleting the My Documents\My games\GalCiv3 folder
3. Deleting the programdata\stardock folder (hidden folder - this will delete any dump files from previous crashes, make a copy and put it somewhere else.)
4. Turn off steam cloud Sync for GalCiv3 (in steam - GC3 properties - Updates)
5. Reinstalling the game.
I think trying to load old save data from custom ships crashes it.