Hi, i have very big problems with these scene:
GI has gone ?!?! What’s going on ???
Every material in the scene → vraymat, glass stuff → vraymat or vrayegzmat…but it still doesn’t work…once it’s rendering right, once it’s rendering wrong 
p.s. vraylog is full of warnings: “Material returned overbright or invalid color (object “set20_kinkiet_23”, RGB: -8,68749 -8,68749 -8,11158)”
Mac
You can send me the scene if you like… BTW are you using any Raytrace shadows for your lights?
Best regards,
Vlado
I have seen something similar when objects with coplanar faces were later added to a scene. lightcache would look fine but then imap would go black.
I get sometimes this grey overlay over my objects looks like mild fog, but its not, and you cant get rid of it, but resetting vray render to scanline and back fixes it.