Corrected two completely different problems, and those are both interesting points to record:

1) It is, in retrospect, not true that an Experia Box V10 means that you cannot set static IPs. At least in Ubuntu you can opt for some sort of hybrid formulation in /etc/netplan, by defining IPs as well as letting DHCP be active. Yippie!

2) In retrospect (again) Drupal 8 was not installed in the most convenient way. You mainly discover this when you have problems updating the Drupal core. I postponed it for a long time, but afterward, it turned out to be simpler than simple. Just reinstalled Drupal 8 with the help of Composer; copied all site-specific folders to the new folders, and modified the symbolic links. And then... then everything ran just as before. Hip hi!