A friend and I were trying to play a multiplayer game and it crashed on startup, regardless of who hosted.
I have a GalCiv3 Elite Founder's license and a secondary regular one that I got on sale. I decided to try and troubleshoot the problem using two computers with the different accounts side by side.
The issue is evidently caused by one (or more) of the DLCs. I registered my secondary account to get the free DLC and tried it. The game was stable at startup with that one DLC enabled on both accounts. I didn't play past a few moves, but it stayed connected until I abandoned the game.
I didn't try the other DLCs because I don't want to spend $10 just to troubleshoot this problem.
So those who are having crashing issues can try disabling one of the premium DLCs and then the other and we'll know which one is the problem child.
In the meantime, disable all but the free one and Bob's your uncle.