Hey des99, thanks for your help. It got me to solve the problem.
So, the thing is, I have the habit of deleting the VST2 versions of the plugins when the installer doesn’t allow me to choose which formats I want to install. When you suggested the problem is related to the installation I re-installed one of the plugins, kept the VST2 file, and let Logic perform the validation which now worked.
It also started working in Studio One, which showed the KORG plugins with no GUI, even when I dragged the VST3 version.
Admittedly, I should be more careful about deleting the VST2 files, but it never caused me trouble before, and I would never thought that deleting VSTx files could affect the AU versions, since in my mind they were separate code.
I conclude from this, that the AU and VST3 versions of the KORG plugins are nothing more than wrappers for the VST2 version and they don’t even contain the VST2 code inside their files - they require the presence of the VST2 file to run. Interesting stuff…
Anyway, thanks for your help, you got me on the right track!