I just ran into this problem too :headbang: . Weird thing was it only happens on certain files and only in 64-bit. Spent a couple hours going through the mel code to figure out why but with no luck. Another wierd thing I noticed is that once you try to view the render settings on the corrupt file, you have to restart maya before you can view the render settings on non-corrupt files.

The solution I've found is to start up maya with a new scene. Then change the render settings to use mental ray. Then import the corrupt file and save the scene . It's important to change the render settings to MR before importing the corrupt scene.