[llamabeast]

Okay, so the problem in the end was that all the pretender files had somehow become corrupted in such a way that Dominions still recognized them, but would crash if it tried to open them. Even having one of these files in newlords (which I did later on in the evening) would cause the game to crash as soon as an age was chosen (so it wasn't that my copy of Dominions had become corrupted after all). Utterly bizarre.

Eventually I just re-downloaded the pretender files from my server and it all worked swimmingly.