This has caused me three days of headaches:
I'm just now able to start rendering a scene (originally made in Maya 2014) in Maya 2015 because it appears that if I have a plane 'visible', Mental Ray refuses to render the scene (exits with Status 1, and reports "'attempt to access null DB tag" then crashes.)

I'm using Maya 2015 Ext1 SP6 with MR for SP6 after a full reinstall over the past three days trying to resolve this. The scene is simply a polygon plane, basically flat but with some small variations on it so it isn't just perfectly flat, and then it has the "Use Background" shader applied because it's only there to catch shadows. I put some paint effects grass and a few trees on the plane, and that's it, but the plane itself isn't actually there for anything but shadows. This scene will later be composited with a background image so it will look like you're looking at mountains in the background with the trees and grass in the foreground. Boring and not complicated. I'm using Physical Sun and Sky, and a fill light, but nothing really fancy.

The problem was/is that the instant I hit batch render, Mental Ray starts and almost immediately goes to "Rendering Completed"--but nothing was actually rendered. Earlier something was causing it to actually crash and freeze Maya, but after basically recreating the scene in Maya 2015 that seems to be resolved. Tonight then, after a lot of work, I had the scene to where it would render completely in the Render View window, which led me to believe that it was finally sorted out. Unfortunately, as soon as I tried to start a batch render again, MR again jumped to "Rendering Completed", and died..again..

But after a bit of checking, I've proven to myself that it's that imported plane. I'd exported that plane previously as a maya binary file, and if I have that plane 'visible', MR will die with the 'rendering completed' statement.

To test all this, I created another simple plane and applied "Use Background" material--and that one rendered fine! Weirder still, if I apply a different material (blinn, lambert, anything) to the original problem plane, it will render fine. But if I have "Use background" material on that original imported plane...or even delete and reapply the Use Background material in Maya 2015, MR will die. I'm very confused why a new one will work, but the other plane I imported makes MR die. It was essentially just there to catch shadows, but MR refuses to run if I have visibility turned on for that original plane.

As I type this, I have the scene rendering away because I just turned off visibility on the ground plane. I lose the shadows of course, but the grass hides it enough I can live without it. I may go back and just recreate a working plane for the shadows and see if that will finally work. But still, I'd love to know why Maya 2015, or Mental Ray in 2015, would ever choke on a simple polygon plane imported from 2014.

I'll say a prayer for anyone who can tell me what is causing this, what MR problem or bug may exist in 2015, or suggest a way to resolve whatever problem is being imported with my original plane.
Thanks..sorry this got long.
JTT