Ok....after a couple of hours thinking I moved the "Vst 3-part folder" inside the Audition folder. After that everything worked.
Hmm... my experience of cause and effect (and a lot of experience with VSTs in Audition) indicates that these facts are very
unlikely to be related.
In general, moving extra folders into the Audition folder really isn't a very good idea, as they aren't a part of the installation. And I've never come across anybody who's had a failure with Audition accessing VSTs - once the correct location to them was established. Most people just use the Steinberg default, but specifying a different path works just as well.
So if it's working now, and it wasn't before, I think you'd have to be looking for another explanation.