Unable to establish secure connection over FTPS to the Plesk server with the hostname example.com.
One of the following warning messages 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
Certificate for securing Plesk is not installed or not valid.
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.