You are right, it’s working indeed! I forgot to enable it after cloning the git:
So for everyone: first clone the app into your owncloud apps-folder, mostly /var/www/owncloud/apps
sudo git clone https://github.com/owncloud/user_ldap.git- and activate it with:
sudo -u www-data /var/www/owncloud/occ app:enable user_ldap
Et voilà LDAP is working again. I wonder why it needed several days to find this simple workaround 