I have a problem with materials preview in Cinema 4D. If I create a material with bitmap texture, the texture doesn't show in the material thumbnail. The same happens when I convert standard material to Octane material.
Example below: here I've converted standard materials (on the right) to Octane materials (left). As you see, Octane materials don't have textures visible.
I must note that I didn't have this problem before. I cannot say for sure when this problem appeared. Probably, after a certain update.
