The error is shown when connecting to a server via FTP:
CONFIG_TEXT: The server's certificate is unknown. Please carefully examine the certificate to make sure the server can be trusted.
Valid from: xx/xx/xxxx
Valid to: xx/xx/xxxx - Certificate expired!
Expired certificate for a server.
Renew certificate for the Plesk server in Plesk > Tools & Settings > SSL/TSL certificate
For example, if it is Plesk self-signed certificate:
- Open Plesk > Tools & Settings > SSL/TLS Certificates > Add
- Fill in the necessary information.
- Click on Self-Signed.
- In Certificate for securing Plesk click Change and select the new certificate.
Also it is possible to use Let's Encrypt extension via Plesk > Tools & Settings > SSL/TLS Certificates > Let's Encrypt as described at How to secure Plesk login page with SSL certificate article.