1. Can you completely remove diffuse for this material? (either set diffuse level to 0 or color to pure black) Or maybe it is important for this material to have it?
2. There was a bug in Corona 1.5, and the solution is to download the newest version (1.5.2), open your scene, and click "reset settings" in "scene" tab. This will revert UHD Cache settings to safe defaults (which are NOT accessible from the ui otherwise).
3. If there are still some problems, it would be great if you could share the scene with us.