OK, I finally figured it out ! 
(with the help of another guy in the forum and some googeling)
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First issue: DNS
My UCS server creates a DNS record in my windows DNS during the initial setup and the domain join (actually there are two DNS records but I donât know what the other one is for.)
Unfortunately it did not update the record after I changed the IP address from a DHCP to a static one.
Nextcloud uses the fqdn to connect to the local ldap server of the UCS and therefore fails to login any users. After correcting the dns record manually I was able to log in -
Second issue: Administrator
I finally was able to log in to Nextcloud but only with a regular user and NOT with my builtin Administrator account. Nextcloud UI always said âwrong passwordâ
I was able to fix that by giving the administrator account a firstname, a lastname and a display name in my windows active directory. Iâve seen this behavior with a different linux appliance that used ldap to connect to a windows AD. -
Third issue: Administrative rights
Despite of what univention claims in the app catalogue, my windows domain administrator is not a pre configured nextcloud admin by default. So I was able to login with my administrator account but in nextcloud it was just a regular user with standard user rights. To fix this I logged in via SSH and looked in the admin secret file
cat /var/lib/univention-appcenter/apps/nextcloud/conf/admin.secret
This seems to contain the password for the builtin nextcloud admin (ânc_adminâ) account in plain text.
With that I was able to login to nextcloud with admin rights and give my windows domain administrator also admin privileges
If you ask me, these issues will be reproducible in any UCS setup with a windows domain and nextcloud
They should be considered as BUGS 
And it should be in the interest of univention to fix them.