I have my install resolved.
First I tried to install Nextcloud on a clean VM in Virtualbox. This went flawless.
Then I had a look at GitHub - NethServer/nethserver-nextcloud: NextCloud integration in NethServer
I saw that the backup also included /usr/share/nextcloud/config/config.php
I remember I did not remove that file when I first installed nextcloud.
So I first de-installed nextcloud:
rpm -e nethserver-nextcloud nextcloud
Then dropped the nextcloud database:
mysql -e “drop database nextcloud”
Then removed the nextcloud directory:
rm -rf /var/lib/nethserver/nextcloud
and removed the nextcloud config:
rm /usr/share/nextcloud/config/config.php
After that installed again:
yum --enablerepo=nethserver-testing install nethserver-nextcloud
And I had a nice loginpage for nextcloud at server.domain.tld/nextcloud.
I also checked if I could log in with a Samba4 account and that worked like a charm.
Great work on the module @giacomo!