Android AsyncTask seems to stop after onPreExecute

I've some problems with AsyncTasks in Android. I searched for hours for a solution but I don't get it... The Task should check via HTTP an account exists and safe an access key via SharedPreferences.

Here is the AsyncTask code: (not integrated class!)

package de.test.barcode;

import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.HttpClient;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;

import android.content.SharedPreferences;
import android.os.AsyncTask;
import android.view.View;
import android.widget.ProgressBar;

public class LoginTask extends AsyncTask<Void, Void, Void> {

    private String username, password;
    private ProgressBar progressBar;
    private SharedPreferences accPrefs;

    public LoginTask(String username, String password, ProgressBar progressBar,     SharedPreferences accPrefs) {
        this.username = username.trim();
        this.password = password.trim();
        this.progressBar = progressBar;
        this.accPrefs = accPrefs;
    }

    protected Void doInBackground(Void... params) {
        ArrayList<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>();
        nameValuePairs.add(new BasicNameValuePair("username", username));
        nameValuePairs.add(new BasicNameValuePair("password", password));
        InputStream is = null;

        try{
            HttpClient httpclient = new DefaultHttpClient();
            HttpPost httppost = new HttpPost("http://{URL}");
            httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
            HttpResponse response = httpclient.execute(httppost);
            HttpEntity entity = response.getEntity();
            is = entity.getContent();
        }catch(Exception e){
            SharedPreferences.Editor prefEditor = accPrefs.edit();  
            prefEditor.putBoolean("is_using_account", true);  
            prefEditor.putString("username", username);
            prefEditor.putString("password", password);
            prefEditor.putString("access_key", "null");
            prefEditor.commit();
        }

        try{
            BufferedReader reader = new BufferedReader(new InputStreamReader(is,"UTF-8"));
            StringBuilder sb = new StringBuilder();
            String line = null;
            line = reader.readLine();
            sb.append(line);
            is.close();
            String accessKey = sb.toString();
            if(!accessKey.equals("null")) {
                SharedPreferences.Editor开发者_运维技巧 prefEditor = accPrefs.edit();  
                prefEditor.putBoolean("is_using_account", true);  
                prefEditor.putString("username", username);
                prefEditor.putString("password", password);
                prefEditor.putString("access_key", accessKey);
                prefEditor.commit();
            }
        }catch(Exception e){
            SharedPreferences.Editor prefEditor = accPrefs.edit();  
            prefEditor.putBoolean("is_using_account", true);  
            prefEditor.putString("username", username);
            prefEditor.putString("password", password);
            prefEditor.putString("access_key", "null");
            prefEditor.commit();
        }
        return null;
    }

    protected void onPreExecute(){
        progressBar.setVisibility(View.VISIBLE);
    }

    protected void onPostExecute() {
        progressBar.setVisibility(View.GONE);
    }
}

And here are the lines which (should) execute the AsyncTask: (all elements are defined right)

LoginTask login = new LoginTask(username, password, progressBar, accountSettings);
login.execute();

i usually have to executeOnExecutor to get it to work like this

new PlayActivity().executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR); 

see this

Android SDK AsyncTask doInBackground not running (subclass)