Set the correct permissions on the new host:
chmod 600 /etc/ssh/ssh_host_dsa_key /etc/ssh/ssh_host_ecdsa_key /etc/ssh/ssh_host_key /etc/ssh/ssh_host_rsa_key
chmod 644 /etc/ssh/*.pub
And restart the ssh daemon. ''Caution'': do not close your existing ssh session as long as you are not sure the ssh daemon has restarted properly and you can login again.
/etc/init.d/sshd restart