Unable to add a server to Plesk 360 with the error: unable to verify the first certificate

Symptoms

• Unable to add a server to Plesk 360 with the error:

  unable to verify the first certificate

• Online SSL checkers (such as Qualsys SSL Labs, SSL Shopper) show one of the errors below:

  Intermediate certificate missing

  ---

  The certificate is not trusted in all web browsers. You may need to install an Intermediate/chain certificate to link it to a trusted root certificate.

  ---

  This server's certificate chain is incomplete

Cause

The certificate consists of 3 parts:

• *.key file private key;
• *.crt - certificate itself;
• *-ca.crt - certificate of Certificate Authority.

The error means the Certificate Authority (ca.crt) part of the certificate is missing.

Resolution

*.key / *.crt / *-ca.crt parts of the certificate should be provided by the corresponding Certificate Authority.

Contact the certificate issuer and ask to provide CA part of the certificate:

• Comodo
• IdenTrust
• Symantec
• GoDaddy
• GlobalSign

Rename the existing certificate under Plesk > Tools & Settings > SSL/TLS Certificates > certificate _name > Rename.

Then, install the certificate again. All three parts should be uploaded/filled for the certificate:

Additional Information

How to secure a Plesk hostname on port 8443 with an SSL certificate (Let's Encrypt / other certificate authorities)