Fatal error with user object in Symfony2 Form Class

Question

I am trying to find an error in my application for days now, maybe some of you can give a hint:

I am using Symfony2, FOSUserBundles and Bootstrap for the forms. In my controller I create within the createAction a form for entering a small ad depending on the user:

$smallAd = new SmallAd();
$user = $this->getUser();

$smalladForm = new SmallAdType();
$smalladForm->setUser($user);
$form = $this->createForm($smalladForm, $smallAd);

After creating an instance of the form I set the user because I need the user ID for the formbuilder. In a former version I set user via constructor, which makes no difference on my local machine.

The form class SmallAdType looks like this (I removed most of the form fields):

public $user;

public function setUser($user)
{
    $this->user = $user;
}

public function buildform(FormbuilderInterface $builder, array $options)
{
    var_dump($this->user->getId());
    $builder
    ...
        ->add('user', 'entity', array(
                'class' => 'AppBundle\Entity\User',
                'query_builder' => function(EntityRepository $er) {
                    return $er->createQueryBuilder('u')
                    ->where('u.id = :id')->setParameter('id',$this->user->getId());
                },
                'label' => 'User',
                'required' => true
        ))
        .....
        ->add('save', 'submit', array('label' => 'Save the Ad'));
}

My problem: on my local develop environment everythings works fine, on the prod server I get the following error message:

int(2)

Fatal error: Using $this when not in object context in   xxxxxxx/Form/Type/SmallAdType.php on line 74

the line with int(2) comes from the var_dump($this->user->getId()); function and is in this case the correct value from my user. I can dump also the complete user object which means for me, that it exists at this point.

I checked the code and configuration on my local and the production server a few times and I can find no difference. Furthermore I don't have any idea where the look now. Any advice is highly welcome.