Oh bugger I've gone over the light limit again.
And now I know what the problem was. I had 719 lights, and added 9, which should be below the limit (736). But these were all animated (which I'd converted from static). I set the Light property to 0, rather than removing it. But the property only needs to remain if the lights are being cloned, otherwise it'll be re-added to the clone, getting its values from the archetype.
So for each light I had Light and AnimLight, meaning each was counted twice.
After removing it, the light count is now 721, on account that I'd also converted existing static lights to animated.