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