The certificate has indeed been signed by StartCom (ordered at
https://startssl.com/ and signed by their Class 1 Intermediate certificate).
I'm not sure why you get these warnings. Anyway i plan to purchase a more recognized certificate in the near future.
StartCom CA is in the OS X system keychain (latest release of OS X). Chrome is not at fault because Safari has the same error. Works fine on iOS and Linux though. Curious.
Indeed, I made some adjustments to the certificate bundle appended to the server one. Do you mind checking again ?