Use same form on two different pages (URLs) in Symfony 2

I am using the FOSUserBundle with Symfony 2.5. I have overridden the default registration form and defined a new URL for it.

fos_user_registration_register:
    path:      /user/new.html
    defaults:  { _controller: FOSUserBundle:Registration:register }

and this is the form

class RegistrationFormType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->remove('plainPassword');
        $builder->remove('username');
        $builder->remove("email");

        $builder->add('terms', 'checkbox', array('required'  => true, 
                                                 'mapped' => false, 
                                                 'constraints' => new True(array('message' => 'Required!'))));

        $builder->add("email", "email", array("required" => true, "mapped" => true, "constraints" => new MailBlacklist()));
    }

    public function getParent()
    {
        return 'fos_user_registration';
    }

    public function getName()
    {
        return 'my_user_registration';
    }
}

finally I registered it as a service.

When I show the form on the URL /user/new.html and submit it, it works: validation errors are shown and if everything is fine with the data, the user is created.

But when I show exactly the same form on a different URL (the root) and submit the form, no validation is done and the user is not saved. Instead, I get redirected to /user/new.html and see an empty form.

I create the form like this

$form = $this->createForm('my_user_registration')->createView();  

Why isn't this working? Is FOSUserBundle checking the referer or something?

My template

<form action="{{ path('fos_user_registration_register') }}" {{ form_enctype(form) }} method="POST">
        {{ form_widget(form) }}
        <div>
            <input type="submit" value="Registrieren" />
        </div>
</form>

Would be great if someone could help my with this "little" problem :)