With a great sense of relief I completed the migration of the website ajna.erbenet.nl tonight. It certainly wasn't a "classic migration" along the lines of exporting the website, switching to the new environment, and then importing the website again. It kept crashing with errors. Finally, all nodes from the old sites were converted manually (i.e. via the "source code" button). That took a lot of time and aggravation. But it worked! Drupal-8 is now completely removed from the website server. There are only Drupal-9 and WordPress left and... oh yes... also some Drupal-7 for the webshop. That will be the next big project: bringing the webshop under Drupal-9.