How do I troubleshoot UNTRUSTED event logs?

I have encountered a web site (ps.meridianschools.org) that I can't access through HTTPS Inspector. The event logs record the connection as UNTRUSTED, which I understand means the certificate presented by the site isn't trusted by Untangle.

I can get the site to work by checking the Trust All Server Certificates option, which is not a good solution.

I have tried different combinations of bypass rules, without success.

This site works fine if I turn off HTTPS Inspector.

I assume this means the site is using a certificate authority that Untangle doesn't trust (but my browser does). Maybe I need to add a root cert to Untangle?

How do I go about troubleshooting this?

I do have a support ticket open for this particular site, but I'm looking to start a conversation here about troubleshooting techniques.