QWidget: Cannot create a QWidget // Maya exited with status 1

Yesterday I've seen this error, for the first time, using batch rendering with Mental Ray. The problem was a plug-in (Maxwell Render plug-in for Maya 2011) never used in my scene but oddly loaded opening the file. The plug-in could not be unloaded because of a reference to maxwellpreviewdata. If I "forced" Maya to unload the plug-in, I got an error that stated:

Plug- in cannot be unloaded because it defines a MPxData derived type that is still in use.

Uninstalling the plug-in is the easiest thing to do, it's a temporary solution that fixes the problem, allowing Mental Ray to work fine again.
As a matter of fact, if I reinstalled the plug-in, the problem came up again.
Next I tried to delete unknown and locked nodes, running the MEL commands:

1. lockNode -lock off `ls -type unknown`;
2. delete `ls -type unknown`;

to clean up the file but it didn't work, Maxwell Render Plug-in was loaded again opening the scene.
The last step I made was a "brute force attack" to the scene not to waste my time.
I saved the scene as Maya Ascii file, opened it with Wordpad, deleted the line:

requires "Maxwell Render 2.0.39"

and saved the ascii file.

It worked, the plug-in isn't loaded opening the file and Mental Ray can melt again my cpu. This solution is very rough and I'd like to find out a more elegant fix.

Anyway I hope to be helpful.

Best regards.