we got several reports that the installation of the Mental Ray products on Windows systems may fail, or may not start at all. It turns out, that this happens when the latest NVIDIA drivers (version >390) have just been installed before the plugin.
The reason is: the NVIDIA installer framework keeps some files on the system, in order to speed up subsequent installations, and to retain a history. The latest installer version coming with the most recent drivers is not compatible with the older Mental Ray installer version, unfortunately. We are working on a fix. For a quick workaround, please visit our forum post Mental Ray Installation Fails.
Sorry for the inconvenience, and Thank You for your patience.