Yeah, I know my posts were a bit vague. The trouble was I was trying to narrow down the problem and failing dramatically. Different map - still broken. Different pretenders - still broken. Suddenly I could only create single player games and no network games at all without getting the same bug. Then they stopped working too! Seriously, it was freaky.
I am going to reinstall and patch from scratch, as suggested. Hopefully then I will at least be able to narrow down the problem. Also it hadn't occurred to me to turn debugging on. I will do that, and that should also help to pinpoint the problem.
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.