I think you are on to the right track, caulbox! I've done some of the same experimenting with similar results. I think the problem also involves the naming (and internal naming) of lights which is not consistent from set to set. A possible conflict occurs when Poser inadvertently creates the same internal name for two different objects - even though the lights show up in Poser preview, they freeze at render time. It might also involve the shadowcam name and data that causes the conflict.