Ok, if anyone is interested my Groove Agent issue is now solved.
I was advised to download a little app called TeamViewer from MySteinberg, which allows Steinberg Support to remotely access my computer over the internet. Within a few minutes Jan Riesener (support) had my Groove Agent working correctly. And the solution was quite simple once you know what to do, so I’ll briefly explain here for the benefit of anyone else who may encounter the same issue:
Jan removed certain Groove Agent 4 64bit folders, restarted Cubase and then re-opened Groove Agent, which caused Groove Agent to do a re-scan.
Problem solved. Thank you Steinberg support.