Not possible to connect server to Plesk 360: The server has an invalid certificate

Symptoms

• Unable to add a server to Plesk 360 with one of the errors below:

  PLESK_ERROR: The server has an invalid certificate

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  PLESK_ERROR: Hostname/IP does not match certificate's altnames

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  PLESK_ERROR: A self-signed certificate is installed

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  PLESK_ERROR: The certificate has expired

• The hostname without a valid certificate or the IP address (i.e. https://213.0.1.2:8443) is used to connect the server to Plesk 360

Cause

Plesk URL used to connect the server is not secured by a valid certificate.

Warning: The IP address can not be secured by a certificate in Plesk.

Resolution

• Check if any related hostname is secured with an existing certificate under Tools & Settings > SSL/TLS certificates > Certificate for securing Plesk.

  Note: That might be verified by using external resources as well - such as Qualsys SSL Labs, SSL Shopper.

  If the certificate is presented and valid, use the related hostname:

  • specify https://server.hostname.com:8443 on adding the server to Plesk 360
  • access the Plesk UI via https://server.hostname.com:8443 and add the server to Plesk 360 from there

• If there is no valid certificate, use the below article to secure Plesk URL:
  How to secure a Plesk hostname on port 8443 with an SSL certificate (Let's Encrypt / other certificate authorities)