Unable to establish a secure FTP connection to a Plesk server: The server’s certificate is unknown

Symptoms

Unable to establish a secure connection over FTPS to the Plesk server with the hostname example.com. One of the following warning messages could be found using the FileZilla FTP client:

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

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Certificate expired!

Cause

The certificate for securing Plesk is not installed or not valid.

Resolution

As the certificate used for securing Plesk is also used for FTPS connection to the server, perform the following:

1. Log into Plesk.

2. Install/renew a certificate for securing Plesk.

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

Additional Information

For Plesk for Windows check the following article.