I finally figured out what caused the problem. It was a config file ```config/autoload/override.local.php``` with some test config in it. Only this array in it is enough to make the ```form_elements``` config key useless:
```php
return array(
    'di' => array(
    ),
);
```
It took me many hours to find out what huge problems this small file caused. Is this intended behaviour? If not, I will open an issue for this.