Bug: VST inserts not loading 3rd party plugs

I have a Cubase 9.01 session that seems to have a bug with 3rd party plugs not loading on the mixer inserts.
The stock plugs load, but any of my 3rd party ones (UAD, Waves etc) nothing happens.
The session has lots of 3rd party plugs already loaded, so something must have changed.
The same sessions works perfectly on 8.5.
Non of the plugs I’m talking about are 32 bit or blacklisted.