Apache Oltu OAuthClient.accessToken() fails with timeout

I'm trying to login users using Google OpenID.

When I try to authenticate a user, I always run into a timeout when trying to retrieve the AccessToken.

public class TestRun {
    public static void main(String args[]) throws OAuthSystemException, OAuthProblemException {
        OAuthClientRequest request = OAuthClientRequest
                .tokenProvider(OAuthProviderType.GOOGLE)
                .setGrantType(GrantType.AUTHORIZATION_CODE)
                .setCode("")
                .setRedirectURI(Env.REST_API_LOCATION+"/login")
                .setClientId(Env.CLIENT_ID)
                .setClientSecret(Env.CLIENT_SECRET)
                .buildQueryMessage();

        OAuthClient oac = new OAuthClient(new URLConnectionClient());
        OAuthAccessTokenResponse response = oac.accessToken(request);
        System.out.println(response.getAccessToken());
        System.out.println(response.getExpiresIn());
    }
}

I always run into a ConnectException (timeout) when oac.accessToken(request) is called and don't get any more info about what's happening.

So, the problem is simple. The system on which my software runs is accessible from the internet, but outbound traffic is forced to use a proxy. As the proxy isn't a transparent one, I had to write my own implementation of HttpClient which is able to deal with a proxy and replace URLConnectionClient with it.