Symptoms
Can't establish a secure connection over FTPS to a Plesk server; errors like these appear
The server's certificate is unknown. Please carefully examine the certificate to make sure the server can be trusted
Certificate expired!
Cause
The certificate used to secure Plesk is either not installed or invalid. Plesk uses the same certificate for the panel and FTPS connections.
Resolution
Install or renew a certificate for the Plesk panel.
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Log in to Plesk
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Install or renew the certificate
Note: Only the connection to the Plesk hostname will be secured. Connections that use other domains require a multidomain certificate.