I've got this error just after hit the render button from the top bar or in the render settings dialog, but everything works fine is I press F9. Same thing if I press the "render!" button in the VFB.
Started working few minutes on a huge vray scene, converted with the script in max 2012, then after doing some test renders and modify objects/lights/materials the error appear. First time appear after changing the environment in the render dialog, then I loaded an autobackup, second time after editing some geometries.
Bye