It happens after any extended period of play on any challenges. It happens most often when starting up combat but can happen at any point. Sometimes graphical elements will turn into a block of white shortly before the crash. It’s as if GSB has an uptime limit and when it reaches it it crashes.
I guess that this may have something to do with the open files bug that has been previously mentioned. I.e. that GSB keeps files open for increased speed and when it hits a Windows limit on the number of open files it crashes.
Since GSB saves state so well, this is less of problem than it would otherwise be. If I spent considerable time doing a custom deployment for a challenge and started up combat and it crashed and lost the deployment I would be peeved. But what actually happens is that I just restart the game and the deployment is there so it’s not such a big deal.
The biggest problem is that sometimes it looks as if I lost to a challenge when I was actually winning and the game crashed. I had one challenge where I beat it and sent the retaliating fleet, but then the game crashed before I could exit and on the challenge list it shows one attempt and no victories.