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 server is added by using the IP address or by hostname without a valid certificate

Cause

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

Resolution

1. Check if any related hostname is secured with an existing certificate under Tools & Settings > SSL/TLS certificates > Certificate for securing Plesk. If so, use it to connect to Plesk 360.
2. 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)