Ubuntu 16.04.2 - Connection reset by peer when ssh right after reboot

I was able to ssh to a Ubuntu 16.04.2 server before reboot. So after reboot I tried to ssh, but failed with message ssh_exchange_identification: read: Connection reset by peer.

Weird part is that, after wait for 10 minutes or so, I am able to ssh to the server. Is there a time lock? If so how to reset that?

Below are my configs on the server:

• sshd: ALL in /etc/hosts.allow

• empty in /etc/hosts.deny

• firewall is inactive ufw status Status: inactive

• ssh to server as root using public key

• enabled the following in /etc/ssh/sshd_config

  PermitRootLogin without-password

  PubkeyAuthentication yes

  StrictModes no

  PasswordAuthentication yes

• In /root/.ssh/config I have the following:

  Host *

  StrictHostKeyChecking no

• client's pub key is in server /root/.ssh/authorized_keys

A detail log is here:ssh -vvv root@192.168.0.100OpenSSH_7.2p2 Ubuntu-4ubuntu2.1, OpenSSL 1.0.2g 1 Mar 2016debug1: Reading configuration data /root/.ssh/configdebug1: /root/.ssh/config line 6: Applying options for *debug1: Reading configuration data /etc/ssh/ssh_configdebug1: /etc/ssh/ssh_config line 19: Applying options for *debug2: resolving "192.168.0.100" port 22debug2: ssh_connect_direct: needpriv 0debug1: Connecting to 192.168.0.100 [192.168.0.100] port 22.debug1: Connection established.debug1: permanently_set_uid: 0/0debug1: identity file /root/.ssh/id_rsa type 1debug1: key_load_public: No such file or directorydebug1: identity file /root/.ssh/id_rsa-cert type -1debug1: key_load_public: No such file or directorydebug1: identity file /root/.ssh/id_dsa type -1debug1: key_load_public: No such file or directorydebug1: identity file /root/.ssh/id_dsa-cert type -1debug1: key_load_public: No such file or directorydebug1: identity file /root/.ssh/id_ecdsa type -1debug1: key_load_public: No such file or directorydebug1: identity file /root/.ssh/id_ecdsa-cert type -1debug1: key_load_public: No such file or directorydebug1: identity file /root/.ssh/id_ed25519 type -1debug1: key_load_public: No such file or directorydebug1: identity file /root/.ssh/id_ed25519-cert type -1debug1: Enabling compatibility mode for protocol 2.0debug1: Local version string SSH-2.0-OpenSSH_7.2p2 Ubuntu-4ubuntu2.1ssh_exchange_identification: read: Connection reset by peer

Thank you!