I keep on having this problem. I receive a model done with 3ds Max + V-ray and the first thing I do is setting the original file as Read-Only.
Working with a copy, I press F10 and “Render”, see how a nice image starts forming seconds later. I may click on “Stop”.
Afterwards, I try to render again and all I get is either a full black or a full white image.
It seems that V-Ray is stateful, it saves the previous state somewhere (filesystem? Registry?). The only possible solution is to go back to the original, read-only copy and start from scratch.
-Ramon F Herrera