I can confirm I have the same problem in Cubase 9.5. Some plugins just won’t load, no message, just red outline over the insert’s tab. And what’s even more stranger is that some plugins sometimes load, and sometimes they won’t. Right now I have a problem with plugins that worked completely normal couple of days ago. And it seems right now I only have a problem with mastering plugins.
Voxengo Elephant is VST2 - won’t load
TDR Kotelnikov is VST2 - sometimes loads, other times won’t
FabFIlter Pro-L - VST2 instance loads, VST3 instance won’t load
They worked before always. Now they don’t.