Selenium server paralel tests error: invalid session id

Hi can somebody tell me please what am I doing wrong in running parallel tests on Selenium server?
I have this simple parallel tests in the class:

package tests;

import categories.Category1;
import com.google.code.tempusfugit.concurrency.ConcurrentTestRunner;
import org.junit.Before;
import org.junit.Test;
import org.junit.experimental.categories.Category;
import org.junit.runner.RunWith;


@RunWith(ConcurrentTestRunner.class)
@Category(Category1.class)
public class ParalelTest extends Base
{
    protected String siteUrl = "/waitforit.php";


    @Before
    public void setUp()
    {
        driver.get(baseUrl + siteUrl);
    }


    @Test
    public void test1()
    {
        System.out.println("test1() thread name: " + Thread.currentThread().getName());
    }


    @Test
    public void test2()
    {
        System.out.println("test2() thread name: " + Thread.currentThread().getName());
    }


    @Test
    public void test3()
    {
        System.out.println("test3() thread name: " + Thread.currentThread().getName());
    }


    @Test
    public void test4()
    {
        System.out.println("test4() thread name: " + Thread.currentThread().getName());
    }
}

and here is the screen with error:

What is wrong with that? Without server it works fine.