Such a message can have various causes. For example, the SSL certificate for the URL of your xentral could be expired or incorrectly set. Also, the server configuration may be wrong, or the libraries may be outdated.
If you get this message, contact the server administrator or the server hoster if the server is hosted externally. They can create or modify SSL certificates and make the appropriate settings.
If you are hosted on xentral Cloud with AWS, please contact us directly at firstname.lastname@example.org.