OK I have some more info.
It seems that this is a known (but very recent) issue caused by the latest Pace toolkit, plugins built with that version are locking up Cubase / Nuendo VST3 scanner. (And others, I also had the same issue with Luna fwiw).
This is why you will probably find (like me) that it only happens on newer versions of iLok protected plugins, (that were built using the iLok latest toolkit.)
So, I guess there’s not much more that can be done right now except, roll back to slightly older versions of the plugins and wait for the people at iLok and various Plugin Devs to sort it out.