Photon Custom Authentication POST HTTPS

We got an issue with the HTTPS connection using POST and custom authentication.

While our custom authentication is working fine with POST and HTTP it will not work with HTTPS, our configuration looks like this:

<AuthProvider Name="Custom"
                    AuthenticationType="0"
                    AuthUrl="http://domain.com/api/authenticate"
                    RejectIfUnavailable = "true"
                    secret="mycode" />
    </AuthProviders>

as soon as we change "http://domain.com/api/authenticate" to "https://domain.com/api/authenticate" photon will not sent any request to our NGINX Server, and our application does not register any request - while it works fine again if we change HTTPS back to HTTP.

The authentication api is accessible on HTTPS and HTTP.

Regards
Christoph