Have you tried resetting your render settings? (Render Setup > Scene > Reset settings) There was a similar report recently, and it turned out the user accidentally changed some setting, and resetting to defaults immediately fixed it.
Other than that, this may be caused by using an HDRI as the environment override (reflections). Unfortunately this is currently a limitation of Corona, but we will fix it in the upcoming versions. In such cases the workaround is to restrain from using environment overrides, or to use a low dynamic range image instead of HDRI (e.g. JPG).
