I think this means the certificate sent by the server isn't trusted / known to your client.
When it works in the browser did you "accept the certificate for this session" or similar, if that's the case then it's definitely because servers certificate isn't trusted by your client.
If this is the case, you can either get the certificate from the server, and import it into the "cacerts" keystore in your jdk, or as a more amusing hack you could write a X509TrustManager implementation that always succeeds, from there, you can force your new AlwaysTrustManager to be used by the SSLContext, which can then provide the default SSLSocketFactory for HttpsURLConnection.
Something along these lines I would imagine.
I have no java certifications. This makes me a bad programmer. Ignore my post.
subject: trying to open a connection to a secure server (unknown certificate)