Bad grey staircase rendering of 3D objects

I am fighting with a problem occasioning bad shadow rendering with 3D objects.

My project is fairly big and contains many Groups of many 3D objects, most of them created with Fox’s famous ZZSC Basis Shapes coma, colon or whatever convenient shape character. Everything ok so far. Tx to Fox's hep.

The trouble comes with the rendering of some of these objects, after disabling all other objects.

It seems the Light parameter is the trouble maker, since I disabled all of the rendering parameters Shadow, Reflection, Depth of Field, Motion Blur, Field Rendering and Frame Blending, and still get ugly grey staircases all over.

I just kept the Lighting parameter on, and I still get the trouble, ie. Square black/grey blob quickening rearranging themselves with my camera movement. That makes the picture unusable of course.

Adding some mild light object makes the trouble somehow better but still not well enough.

My question is, how can  deal with this bad rendering problem?

Motion 5.9, project 60 fps, 600 frames.

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Claude