Render nodes don't load XRefs

[AltoR]

My problem is gone: the scene now renders fine with DR On.
There was a problem with the main scene, not with the XRef scenes as I first thought.
I started from a new empty scene. I put the XRef Scenes in it. The VRay DR worked fine. Then I added the main scene and it still works fine.

Thank you for your support,

[tashko.zashev]

Actually we didn't manage to reproduce the black renders at our side and this makes me thinking that the issue is related to one of the missing files. However, thanks for posting the solution you found it will help other users that may experience similar problems.