Ah, I think I've found the problem. It's the sprite for the Saxon Horde Raider unit, there was a typo in the descr_model_battle.txt file. I tested all the units before sprites, and all but the raider afterwards.

Open the descr_model_battle.txt file and replace:

model_sprite saxons, 60.0, bi/data/sprites/saxon_raiders_sprite.spr

with:
model_sprite saxons, 60.0, bi/data/sprites/saxons_raiders_sprite.spr


Hopefully that will fix the crashing problem, as it was enough to cause BI to crash everytime the raider was used.

I'll update the existing files, then I'm off to bang my head in frustration at seeing another bug creep into things.