That error is usually due to a misconfigured certificate or a misconfiguration with the webserver config files as it relates to certificates… I suggest going to this web page and checking your certificate for issues.
https://www.digicert.com/help/
By the way, here are the most common reasons for that error:
The CA that issued the server certificate is not known.
The certificate was self signed rather than signed by a CA
The web server config does not have an intermediate CA