Okay. It may take me a little while to do that - it happens seemingly at random times so could be a while into the game. After clicking [OK] the game crashes to windows altogether so there's not much of a game file left. I'll try to save after every turn and send the last saved game before it gets the error.
And, yes it is a mod with all its own bitmaps and scripts. Not sure how much is actually necessary to send - I suspect it's related to the AI purchase scripts. I've learned since I posted this that if I remove the AI scripts, I never get the error.