I ran the Qualsys SSL Labs test on the site[0]. It reports an incomplete chain; that is, the server doesn't supply the intermediate certificate required (Entrust Certification Authority - L1C) to bridge the gap between the site's certificate and the Entrust certificate in most browser/OS stores.

This intermediate certificate must instead be downloaded by the browser. I suspect, therefore, that some browsers do not have the intermediate certificate and so the certificate cannot be validated.

[0] https://www.ssllabs.com/ssltest/analyze.html?d=isapps.acxiom...