Error: The renderer mentalRay is not registered yet (cmd line invoke)

[michaelhubbard]

I am getting the following error message when invoking Maya through the command line with a script that sets the current renderer to mentalRay:

// Error: The renderer mentalRay is not registered yet. // 
// Error: Renderer "mentalRay" is not valid //

RuntimeError: Error occurred during execution of MEL script
file: C:/Program Files/Autodesk/Maya2012/scripts/others/unifiedRenderGlobalsWindow.mel line 568: The renderer mentalRay is not registered yet.

I have searched some of the forums which suggest that deleting the userPrefs.mel, which seems to work (I can blow away the userPrefs file each time I run the script if need be), but it just seems like overkill for one (or more) values. Does anyone know which userPref values are causing Maya to not register the mentalRay command when invoked from the command line?

[michaelhubbard]

Through trial and error I managed to find the optionVar in the userPrefs that prevented mentalRay from being assigned when Maya is invoked through the command line. The pref is:

-iv "oneTimeDefaultPluginLoad50" 1 

Does anyone have any insight into what this preference is actually doing, and why it would be causing this issue?