Hey,
this looks like the symptoms described in this thread (in German, unfortunately). The reason turned out to be a difference in DNS resolution inside the Nextcloud container and outside: the host name of the host running the container was resolved to an external (public) IP address inside the Docker container. Therefore the HTTP requests used for modifying the Nextcloud groups were sent to said external address instead of the host running the Nextcloud container.
Please run the following and paste the output:
# This is run on the host itself, outside of the container:
nslookup $(ucr get hostname).$(ucr get domainname)
ucr search --brief '^nameserver|hosts/static'
# The next command enters the container:
univention-app shell nextcloud
# This will now be run inside the Nextcloud container:
sudo -u www-data php /var/www/html/occ config:list | grep -i ldap | grep -v password
apt-get install iputils-ping
ping -c 2 $(sudo -u www-data php /var/www/html/occ config:app:get user_ldap s01ldap_host)
Kind regards,
mosu