Once when I installed version 10.0 MasterRig plugin arrived but then left when I used “Force Plug-in Detection at Next Launch” function (Preference> Plug-ins> General).
For some reason, the Master Rig does no load properly on your system. A conflict somewhere. Hard to know.
But apparently, sometimes it works, according to your above quote.
But when you do “Force Plug-in Detection at Next Launch”, everything is evaluated again.
The best suggestion I can give: install WaveLab on a system where you don’t have the problem. Then copy this file from the working system to your work system:
C:\Users[YourName]\AppData\Roaming\Steinberg\WaveLab Pro 10.0\Cache\plugin-registry.txt