I had to update my certificate for my zimbra server (it had expired) and we renewed again with Digicert. Certificate install went fine and the install tester tool on Digicert reports that it is installed correctly. Accessing our server via HTTPS also shows the cert is installed properly.
However, all my Zimbra Desktop see the certificate as invalid and I must go through the account settings and tell it to use the untrusted certificate. This seems to only be a problem with Zimbra Desktop.