Trying to connect to an openSUSE thumbleweed system via ssh fails. It doesn't
matter if connecting from remote or local. In all cases the server closes the
connection.

Firewall is open for port 22
/etc/hosts.allow holds
sshd : ALL : ALLOW

All server keys generated have length 0 in /etc/ssh! After deleting these keys,
then recreating them the server works. It turns out the keys where not
generated because of an old, deprecated option used. Instead or not creating
keys in this case ssh-keygen creates keys with zero length.