OK, I'm in a cul de sac here. You've got a configuraation similar to mine, so it must work. I remember a similar bug, but I've always managed to get past it, only I don't quite remember how.
Tell you what: Does the error appear during load or save? Try starting a new game just for the sake of seeing if it persists. Make sure that there is no garbage in the Fallout/data/maps directory (to my mind , it should be empty, if it isn't, save games can't extract there.) If all else fails, try reinstall. I can't help you any further, sorry.