Hi all,
Thanks for your reply and thoughts on this. It's been a real head-scratcher. There's always something that gets the brain cells twitching on a normal day, but this has been a proper challenge!
We had a good idea that it wasn't related to phong, normals or generally related to the model geometry as it is all modelled from scratch within C4D and the error wasn't consistent with mesh errors due to the very rectangular and overlapping nature of the areas in question.
However, when we shifted the camera and removed all the textures from the scene, the problem persisted. I eventually ended up collapsing the entire scene in to two meshes and removed all tags. I was left with a bare mesh, a light and a camera. And then I noticed it...! When we prepare photo montages, we add a 'background' object as the child of the camera. This has the backplate mapped on and in our default scene we have a Python Tag that controls the visibility and render-ability of the backplate. When the camera is used, the background becomes visible but, crucially, is not renderable (see attached). This a solid tried-and-tested way of working with numerous backplates in a single scene and something we use daily. However, in this instance, the artist in question removed the Python Tag and the 'background' object 'traffic lights' were grey, so Corona was attempting to render it. As far as I understand, the background object isn't supported / recognised by Corona and therefore it just became an annoying interference that made a right mess of the render output!
Anyway, to cut a long story short... we fixed the issue and will be keeping an eye on "Artist X' to make sure he doesn't ruin my day again.
Thanks again for your responses and apologies to the Corona Team for possibly insinuating that your beautiful software may be to blame... 😳