Unable to retrieve FTP directory listing via SSL/TLS secured connection

Symptoms

• It is not possible to get a directory listing by FTP using SSL/TLS secured connection:

  Error: Failed to retrieve directory listing

• Connection to FTP from localhost on the server itself works fine:

  Status: Connecting to 203.0.113.2…
  Status: Connection established, waiting for welcome message…
  Status: Initializing TLS…
  Status: Verifying certificate…
  Status: TLS connection established.
  Status: Connected
  Status: Retrieving directory listing…

Cause

Firewall/NAT misconfiguration in between the Plesk server and the end-user.

Resolution