Edit: After a little more trying, it seems it's not only when I overwrite but, I guess, simply when I save my game. I just tried using the console function to make a working save. I tried that before as well, one time to test it, then when I overwrote that game, it crashed. This time it crashed when I tried making a fresh save using the console.
Edit 2: Okidoki, figured it out. It will only crash when I overwrite a savegame after I've sold platinum coins (Oscuro's Oblivion Overhaul mod). I guess these seemingly valuable coins are far more trouble than they're worth!
So, uh, thanks for your help. Is there any way to delete this?