EDIT

see https://www.drupal.org/docs/upgrading-drupal/how-to-prepare-your-drupal-7-or-8-site-for-drupal-9/upgrading-a-drupal-8-site

• First update drupal to 8.8 or 8.9, these versions are compatible with the drupal 9 API. (actual version 8.6.10)

• Update all modules to the drupal version.

• Use Composer to manage Drupal dependencies. Drush 9 and newer no longer supports updating Drupal.(see https://www.drupal.org/docs/updating-drupal/updating-drupal-core-via-drush)

• https://drupalize.me/tutorial/update-drupal-versions-prior-88x-using-composer?p=3233

On https://cybertaxonomy.org/node/4/edit I tried to change the link for "Taxonomic Editor (TaxEditor)" from "http://wp5.e-taxonomy.eu/taxeditor/" to "https://cybertaxonomy.org/taxeditor/" in the pop-up dialogue but when saving the dialogue I always get:

"An error occurred while attempting to process /editor/dialog/link/basic_html?_wrapper_format=drupal_modal&ajax_form=1&_wrapper_format=drupal_ajax: ajax.$form.ajaxSubmit is not a function"

Editing text on https://cybertaxonomy.org/setups/group/server_requirements does not work. The text seems to be stored elsewhere. Maybe this is to avoid redundancy.

Problems with url editing is fixed with an update to 9.4.5

There are still folders "/var/www/drupal-8.9", "/var/www/drupal-8" and "/var/www/_drupal-8" on edit-community. Can they be removed or archived?

Andreas Müller wrote in #note-17:

Uploading an image while editing content (in basic html mode) did not work. I always got the following error:

Maybe this is because of right issues, I will have a look.

-> changed the group to www-data (the apache user) and add write rights to the group for the "files" directory.

-> This should be fixed now. I tried to upload an image and there was no error anymore.