Android : HTTPS Post resulting in HTTP 1.1 500 internal server error

i`m trying to login to a HTTPS webside (https://mobiel.ziggo.nl/Account/LogOn) but i always get a "HTTP 1.1 500 internal server error" I need to pass following elements and i use a MySSLSocketFactory class which (for now) accepts all certificates:

• __RequestVerificationToken: token


• ReturnUrl: ""


• Username: "user"


• Password: "pass"

I use following code:

public String postRequest() {
    String responseString = "";
    HttpClient httpClient = HTTPUtils.getNewHttpClient(REST_SERVER_HTTPS_POST_URI.startsWith(HTTPS_STRING));
    HttpResponse response = null;
    InputStream in;
    URI newURI = URI.create(REST_SERVER_HTTPS_POST_URI);
    HttpPost postMethod = new HttpPost(newURI);

    try {
        postMethod.setHeader("Content-Type", "text/html; charset=utf-8");
        postMethod.setHeader("Accept", "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8");

        List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>();
        nameValuePairs.add(new BasicNameValuePair("__RequestVerificationToken", "IYx2o6-W-RcdhyV4sbcoWzOLXmsUrtWeYF8Tqee"));
        nameValuePairs.add(new BasicNameValuePair("ReturnUrl", ""));
        nameValuePairs.add(new BasicNameValuePair("Username", "test"));
        nameValuePairs.add(new BasicNameValuePair("Password", "pass"));
        postMethod.setEntity(new UrlEncodedFormEntity(nameValuePairs));

        response = httpClient.execute(postMethod);
        Log.e("response", response.getStatusLine().toString());
        String aaa = response.getStatusLine().toString();
        System.out.println("RESPONSE: " + aaa);

        in = response.getEntity().getContent();
        responseString = convertStreamToString(in);
    } catch (Exception e) {} 
    return responseString; 
}

What could be wrong? Thx for helping me with this.