Unable to establish secure FTP connection: The server's certificate is unknown

Symptoms

Unable to establish secure connection over FTPS to the Plesk server with the hostname example.com.

The following warning message could be bound using FileZila FTP client:

The server's certificate is unknown. Please carefully examine the certificate to make sure the server can be trusted

OR

CONFIG_TEXT: Certificate expired!

Cause

Certificate for securing Plesk is not installed or not valid.

Resolution

The certificate used for securing Plesk also used for FTPS connection to the server.

Install/renew a certificate for securing Plesk. Refer  How to secure Plesk login page with SSL certificate for detailed instructions on how to secure Plesk with SSL certificate.

Note: Connection will be secured only if the connection goes to example.com. If the connection goes to any domain like domain.tld then multidomain SSL certificate must be used for securing Plesk.