Well I think the issue was really the very low MSR on the very large ground plane. It's kinda sad, though, to know that Poser's renderer will only use 4GB of RAM always, so its no good to upgrade my hardware.

I'll try also some of Baggings tests to see what happens. So far my scene has no actors. It's supposed to be an brazilian indian preparing the ground for planting, so no other app was up to this challenge like Poser. And he can't be naked either as kids will probably see this, so I gotta venture with cloth and collisions... oh boy... well, maybe just a loincloth won't be that hard to do.