Quote - One of the things happening in the "Rendering" step before the first bucket is drawn is the creation of the raytracing acceleration structures. If the scene renders fine without raytracing, that might be an indication of where to look.

All right. I tried to render the same scene without raytracing and it rendered fine, but memory usage stayed below 1.5 GB.
So I added another character to the scene, and tried to render that. Right in the beginning, before starting to render the shadow maps, memory usage went up to 1.5 GB, and Poser 6 gave me the "There has been a problem, please lower memory usage, etc" message... At this point Poser was using 1.5 GB, overall memory usage was 1.7 GB.

So the 1.5 GB limit exists also without raytracing, except that Poser handles it better, detecting there is a problem, and doesn't fall into the infinite loop it does when raytracing is on.