Quote - Erik. Check the direction of your normals. If they are not all pointing consistently in the same direction some of the polys will render black.

That's the first thing that came to my mind too, but how can it be that one (intire) object is black in the first render, another part looks normal, and in the next render it's the other way around?

Besides, when converting my STL's exported from Inventor in MeshLab I (almost) always let MeshLab 'reorient all faces coherently' (if possible), and then 'invert face orientation' twice to see if everything is ok before exporting as OBJ.

Also, some of the affected objects were subsequently decimated in ZBrush, others weren't, so that also didn't lead to any plausible explanation.

Just before these render mishaps I repeatedly had an NVidea driver 'incident' because I was oveloading DAZ Studio with geometry (kind of a torture test). But I cold-rebooted my PC after each crash, so this shouldn't (ahem, well...) have anything to do with the LuxRender f-u-ps I had later. Hmmm, could be a torture-test related thermal issue with the video card, but that's not very likely, it had more than enough time to cool down.

I'll clear all Reality and LuxRender data for this job, and try it once more. See what happens.

Cheers!

Erik