GateKeeper Blocking Audio Plugin

I'm an audio producer although I have also worked as a C/C++ programmer so I'm reasonably technical! I bought a Mac Mini about a year ago and it recently developed a fault and had to be replaced so I'm currently in the process of setting up a new Mac Mini running Catalina (10.15.3). One of the audio programs I use is PreSonus Studio One. A big part of how it's used is via third-party plug-ins which come in a variety of formats - AU, VST2, VST3, etc. The plug-ins are installed in different ways - some via a "control centre" app, some by an installer, others by drag-and-drop to the appropriate folder. I've run into a problem with one VST3 plug-in. I dropped the plug-in into the folder /Library/Audio/Plug-Ins/VST3. Then I ran Studio One which does a plug-in scan at start-up. An error message popped up (can't remember exactly what it was) saying that the software was from an unidentified developer and couldn't be run. Ever since then, the plug-in won't load. I suspect the plug-in has been "blacklisted". I've tried the following in order to fix the problem but so far nothing's worked: - Re-installing the plug-in with an installer - Disabling GateKeeper with: spctl --master-disable - Resetting GateKeeper with: spctl --reset-default Other plug-ins are loading fine. Now I'm stuck. Any suggestions as to what I should try next?