SSH issue – Disconnecting: Received data for nonexistent channel 0

Are you certain that the user account you're attempting to access is correctly configured? If you log in as root on the system, can you su to the user account?

# su - username

What do you see in your logs after a failed connection attempt? On many systems, sshd will log to something /var/log/secure or /var/log/auth.log. Also, I note that you have PasswordAuthentication enabled but ChallengeResponseAuthentication disabled. Do you see the same behavior if you enable ChallengeResponseAuthentication?

Here are some general diagnostic steps to use when you have ssh problems:

• Enable verbose diagnostics in ssh:

  ssh -v host.example.com
  

  This will cause the client to output a variety of diagnostic messages as it negotiates the connection. This will often provide a clue to the problem.

• Run the server in debug mode.

  On your server, stop sshd, then run it from the command line like this:

  /usr/sbin/sshd -d
  

  This will produce verbose debug logging on stderr that will very often contain useful information.

If neither of these helps you figure out what's going on, would you add the output to your question?

You can safely restart an ssh server whilst connected to it. It specifically keeps your connection open to help avoid locking you out. But you should always open a second connection and check that you can initialise a new connection before closing the first one!

It sounds to me like that's one of two problems:

1. sftp-server falling over for some reason (missing libraries, segfault, etc).

2. The server is running a non-openssh sftp-server instead of the openssh version. This always causes problems.

As a quick test, see if you can run the sftp-server binary by hand without it crashing. It should just sit there waiting for input (kill it with ctrl-c). If it complains about missing libraries or it segfaults you'll know what the problem is.