OS: Windows Vista Ultimate SP2, x32
Core2 Duo E8400, 4GB RAM
FreeOrion version: 0.3.1-RC5 [Rev 1964]
Every time I attempt to save my single-player game, the save window disappears with no error message as if it was successful, but when I open the load or save window again, the file I tried to create is not there. The only file present is the autosave file it created when I started the game - AS_Human_0001.sav - and if I try to save over that, and then load that file, what it loads is the original autosave, rather than the game state that I tried to overwrite it with.
I tried saving a game to the C:\ folder to test whether it was a problem with saving to that particular location (C:\Users\Tallinu\AppData\Roaming\FreeOrion\save - the default location) and was rewarded with the server crashing. Apparently that's not allowed.

Then I tried saving in the C:\Users\Tallinu\Documents folder. Again, no file was created.
The game in question was started with 500 stars, in a 3-armed spiral, mature, with many starlanes, high planet density, and uncommon specials.