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If you have such kind of error in syslog

auth|security:err|error sshd[23003268]: error: Could not load host key: /etc/ssh/ssh_host_ecdsa_key
sshd[5898336]: error: Could not load host key: /etc/ssh/ssh_host_ecdsa_key

You need just to configure an ECDSA key:

[root@nim] /root # ssh-keygen -t ecdsa -b 521  -f /etc/ssh/ssh_host_ecdsa_key

Or without passphrase and no confirmation:

[root@nim] /root # echo |ssh-keygen -q -t rsa -N ''