It was a bit stressful and therefore a bad night. This morning quite early (for me) behind the keys again. And the VPS simply does not return a signal. Then try to ping, both from Windows and from Linux. In both cases nothing at all!
DAMN!

That server is still not back online after about ten hours of waiting!

Then pick up the phone. I'm on hold at Strato. But before that, you have to enter your customer number first. So the help desk sees who is on the line, searches for all the information, probably immediately sees the problem, and just presses the reset button. This scenario plays out in my head, but a hard indication is that while I'm on hold, I suddenly see that the VPS starts to respond to the pings. So "something" has come online. I can't log in yet, but the server is back. I put down the phone and waited another half hour. Then it was all there again. With Plesk this time.

This time I have done a better study of Plesk and am taking it step by step. I entered my domain, erbenet.nl, in Plesk as the main domain. It gets as document root /var/www/vhosts/erbenet.nl. Drupal8 installed in that last folder. That doesn't do anything. Just think quietly. In the folder erbenet.nl I placed a link to the drupal root, which is now ./drupal8. (ln -s drupal8 www.erbenet.nl) Tried to call www.erbenet.nl again, and then exactly what should happen does happen: drupal setup is started.

HIP HEY!
Next step: creating e-mail users, hanging under erbenet.nl. To start with richard@ and then the rest. And ping-pong....all kind of e-mail begins to arrive. Apparently, there was a lot waiting for a successful retry.
In short, if I hadn't been so damn impatient I would have had a working setup days ago. A program like Plesk really does what it's supposed to do. It does have its own learning curve, but that's really only good.

HOORAY for PLESK!