Just had this problem as well and its not prefs.

Kept getting this error when switching the render globals to Mental Ray:
// Error: Cannot use data of type no type in a scalar operation. //

Here’s what worked for me:

1) Create a new .ma scene file with some lights in it. (For some reason an empty maya file will NOT work.)
2) Turn on the Mayatomr.bundle plug-in.
3) Toggle render globals button and set to Mental Ray.
4) Import the busted scene file and save out a new .ma scene file (I’ve been saving out with the render global window open.)

After that everything seems solid again. Don’t know why having lights in the original file seems to be the magic bullet, but it works again.

Good luck.