I’ve just solve it.
The key was that first Error, No reachable KDCs were found.
The problem was I’ve only put one nameserver in the Network config, but forgot to add the loopback 127.0.0.1 as another nameserver, according to https://help.univention.com/t/no-reachable-kdcs-were-found/13807/2
That solved the second problem of the list too.
Thanks for your time.