FTP connection does not work on Plesk: FTP 550 SSL/TLS required on the control channel

Symptoms

Unable to connect to FTP server on Plesk:

# ftp domain.tld 21
550 SSL/TLS required on the control channel
Login failed.
Remote system type is UNIX.
Using binary mode to transfer files.

Cause


Allow only secure FTPS connections
option is enabled.

Resolution

1. Log into Plesk
2. Set
Allow both secure FTPS and non-secure FTP connections
in
   Plesk > Tools & Settings > Security Policy