it worked with
sudo -u www-data php occ maintenance:update:htaccess
But that occ-comand isn’t documented anymore:
https://docs.nextcloud.com/server/12/admin_manual/configuration_server/occ_command.html?highlight=occ
Edit 2018/07/25:
Is documented again:
https://docs.nextcloud.com/server/13/admin_manual/configuration_server/occ_command.html?highlight=occ#maintenance-commands