SFTP is not available for additional FTP users on Plesk server

Symptoms

• SFTP connection with subscription's additional FTP user cannot be established with the similar error on connection attempt:

  # sftp additional_user@203.0.113.2
  additional_user@203.0.113.2's password: 
  Connection closed

  # sftp additional_user@203.0.113.2
  additional_user@203.0.113.2's password: 
  unexpected end of file

  Note: SFTP stands for SSH File Transfer Protocol and uses SSH for establishing a connection and transferring the data.
  FTP and FTPS connections are not affected by this issue, as they use different protocols and mechanisms.

• Similar errors might be logged by the Journal:

  # journalctl --since="10m ago" | grep chrootsh
  plesk-chrootsh[16727]: user with id=10000 and name=additional_user not found in chrooted passwd file 
  plesk-chrootsh[16727]: system error: No such file or directory

Cause

SFTP configuration for additional FTP users is not implemented in the same ProFTPD instance as the system users.

Resolution

Since Plesk Obsidian 18.0.80, it is possible to enable SFTP access for additional FTP accounts via a separate ProFTPD instance listening on its own port.

Select the “Allow SFTP for FTP accounts” checkbox found on the “Tools & Settings” > “Security Policy” page.

Optionally, specify the port number to be used for this additional instance.

This feature is enabled by default. To hide these controls, add the following lines to the panel.ini file:

CONFIG_TEXT: [ftp]
sftpManagement = off