I want to write down my findings to help anyone else searching for this topic.
I had a problem on my hypervisor an thus the univention-VM was not responsible any more. I invoked an clean reboot.
After the reboot the VM started to single-user mode (initramfs#) and mentioned me to do manual fsck so I did “fsck -y /dev/mapper/vg_ucs–root”. This repaired some stuff und so I rebooted.
Now the VM started to the normal console, but wasn’t reachable over the network. On the hypervisor display the IP of the Nextcloud container appeared instead of the external IP.
“ip addr show” displayed the nic ens18 down and so I took a look at /etc/network/interfaces. Instead of the nic ens18 there was enp6s18. I temporarily changed the name of the nic back to ens18 and restarted network “/etc/init.d/networking restart” and the system is reachable over the network again.
This ist not the final solution, because after the reboot there is enp6s18 again. But now the system is reachable and I can Access “Univention Configuration Registry”. I did a searching for “enp6”. Here I change “interfaces/primary” from enp6s18 to ens18.
This is not the end of the road, because there are some other UCR-Varibles with content “enp6s18”. Either I have to change these all (and maybe it is not working), or get the system back to recognise the nic as “ens18” as it did before.
I will work further to fix this issue and will tell back.