Short update: I trashed all my preferences (even the ones of the previous versions) but all to no avail.
This seems to be a severe graphical problem with not only the inspector, but also the mixconsole.
It is rendering my Cubase as good as unusable.
If I try to expand for instance the inserts in the mixconsole, nothing happens. But if click on the inserts, close the mixconsole and then reopen it, the inserts are visible.
The same more or less happens in the inspector. If I click on any item in the inspector, nothing happens.
If I try to switch to the visibility tab in the inspector, I first have to click into the space below the tab before the visibility settings appear. The same happens when I switch back to the inspector.