Very interesting discussion ... I have the same grey patches since some days in the upper left corner of each render thread. Equal if using 1, 2 or 4 threads.

They came with a relatively huge model of a building and there mainly if positioning the camera inside of the building but without touching that geometrie. As first I thought of some interference between the memory holding geometric data and render space.

But that it's a shadow of geometries behind the camera seems to me indeed true.

The easies work around I have found is to use P6 if you have it also on the computer.
The same PZ3 with the complete scene and camera position causing the grey patches in P7 will render absolutly clear in P6 ... ok, you loose the possibility to use more than 1 thread, so it's a little bit slower on multi core pc's ...

So let's hope for next SR ...

Joachim