FreeOrion

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.

You're using an ancient version. You can get the latest official release here. There are also recent windows builds available here.