IoS 7.1.2 CFHetworks SSLHandshake error (-9807) on one device only

We have an older "production" version of our application (never released, but our production codebase for testing and demonstration outside of the dev codebase) that is used for demonstration. We had an SSL certificate which expires a week ago. We have renewed the certificate and all devices can connect to our server again except one. No matter what this device gives

CFNetwork SSLHandshake error (-9807) (which I consider to be an "invalid certificate chain")

Over and over when the app tries to connect (via NSURLConnection). All other devices, including those running the same version of iOS, the same version of the app, and on the same network, have no issues and connect just fine. It's just that this device has problems. The device is running iOS 7.1.2 and it was restarted and the app was uninstalled and reinstalled. In addition, the app was tested over both cellular and WiFi connections. Keeps giving this error. (This is the boss phone and it gives demos). One of my test devices has been updated to the same iOS version and works with the same app. He has no connection problem. My personal device is on the same network and app version, but the lower (7.0.x) iOS version has no connection issues.

I'm not sure how to debug this. I haven't tried to rebuild the app yet (the codebase is 6 months old and will need to tweak and test out an old production branch), but was wondering if there are some simple tests or other things I can look at.

The backend server seems to be fine (only with this device and some independent certified testers connected and working fine with the certificate chain).