ssh error

root@server1:~# service ssh restart
Could not load host key: /etc/ssh/ssh_host_ecdsa_key
[....] Restarting OpenBSD Secure Shell server: sshdCould not load host key: /etc/ssh/ssh_host_ecdsa_key

The root cause of this problem is that sshd daemon somehow is not able to load SSH host keys.
When OpenSSH server is first installed on Linux system, SSH host keys should automatically be generated for subsequent use. If, however, key generation was not finished successfully, that can cause SSH login problems like this.
Let's check if SSH host keys are found where they should be.

$ ls -al /etc/ssh/ssh*key 

make sure size is not zero,

good example:-

 root@server1:~# ls -al /etc/ssh/ssh*key
-rw------- 1 root root  668 Nov 30 18:12 /etc/ssh/ssh_host_dsa_key
-rw------- 1 root root  227 Dec  5 15:03 /etc/ssh/ssh_host_ecdsa_key
-rw------- 1 root root 1675 Nov 30 18:12 /etc/ssh/ssh_host_key
-rw------- 1 root root 1671 Nov 30 18:12 /etc/ssh/ssh_host_rsa_key

[SOLUTION - debian / ubuntu ]

root@server1:~# dpkg-reconfigure openssh-server
Creating SSH2 ECDSA key; this may take some time ...
[ ok ] Restarting OpenBSD Secure Shell server: sshd.

[SOLUTION - redhat/centos]

On CentOS, RHEL or Fedora, all you have to do is to restart sshd after removing existing (problematic) keys.

$ sudo rm -r /etc/ssh/ssh*key

$ sudo systemctl restart sshd

An alternative way to regenerate SSH host keys is to manually generate them using ssh-keygen command.

$ sudo ssh-keygen -t rsa -f /etc/ssh/ssh_host_rsa_key
$ sudo ssh-keygen -t dsa -f /etc/ssh/ssh_host_dsa_key
$ sudo ssh-keygen -t ecdsa -f /etc/ssh/ssh_host_ecdsa_key 

Once new SSH host keys are generated, make sure that they are found in /etc/ssh directory. There is no need to restart sshd at this point.

$ ls -al /etc/ssh/ssh*key

Now try to SSH again to the SSH server to see if the problem is gone.