This is more than likely because the intermediate certificates were never installed. Installing them should resolve this error. Below you will find links on where to locate and install your intermediate certificate, depending on the Certificate Authority that issued the cert. Or you can always contact your SSL provider.
- https://knowledge.digicert.com/generalinformation/INFO4331.html
- https://knowledge.geotrust.com/support/knowledge-base/index?page=content&id=AR1421
- https://search.thawte.com/support/ssl-digital-certificates/index?page=content&id=AR1384
- https://knowledge.rapidssl.com/support/ssl-certificate-support/index?page=content&id=AR1548
- https://support.comodo.com/index.php?/Default/Knowledgebase/List/Index/108/sha-2
