Jetty reports certificate_unknown on inbound https request

I’m developing the Rachio binding, which uses webhooks to receive events from the cloud. The clound platform is hosted on AWS and supports https, but I’m not receiving any requests (even the manufacturer confirms and showed that https is working).

After turning on jetty debugging I saw the following error when the event REST calls comes in: Received fatal alert: certificate_unknown

Do I need an export of the certificate in the local key store?
How to enable some more jetty debug output so I get the issuer of the certificate?