VRay Light Gen Thumbnails showing up black

Hi there! I have found when using the v-ray 5 light generator that the thumbnails which should show the different lighting options and HDRs are showing up completely black, meaning I am unable to see the differences and pick one. I have attached a screenshot of what it looks like.

Does anyone know what is causing this? In a new model it works fine, it seems to be in models I have worked on for a while.