[Q]

I switched one of the AI empires to manual control. It was a almost dead empire, who had only some fighters left, which I wanted to eliminate. After I did that, I waited for the message "You loose" and continued with my real empire then saved the game.
When I tried to reload this savegame, it turned out to be impossible.
Went back to the last intact savegame, did everything the same but switched back the dying empire to AI control immediatetly after the fighters were eliminated. Everything was fine.
Conclusion: If one of human controled empires dies (displaying the "you loose" picture), the savegame gets corrupted and you can't load it again.