I modeled an assembly line R20, lit and textured it successfully. Then used the symmetry object to replicate it across the factory floor. In R20, everything looked as expected. I used an infinite light with ambient fill. Standard stuff.
I opened the same model in R21 and the textures and lighting went crazy. Most of the surfaces turned black, and the reflections/refractions in the glass, and reflection from the steel surfaces became totally deranged. The symmetry object created what appear to be very unusual UV mapping/phong effects in which geometry and ngons that were formerly invisible, now appear in the render. Very distressing.
I assume that this is due to changes in the way R21 handles materials and lights. I can’t share the model but am hoping you can diagnose the fundamental misstep from the description. Can you point me to the solution? Is it (again) a missing check box?
I would have sent images but after an hour of trying I couldn’t get them small enough to satisfy the pixel sheriff. Will try again if you need them. My expectation is that the solution is obvious to the trained professional.