Silverstripe Elemental trouble after creating related dataobjects - silverstripe-4

Question

Facts: SS 4.0.1, dnadesign/silverstripe-elemental 2.x-dev, php 7.1 and Zauberfisches Vagrant box Jessy Version 3
I made an Elemental Element that is a holder for single elements:
namespace R12page\Elements;
use DNADesign\Elemental\Models\BaseElement;
use SilverStripe\Forms\GridField\GridField;
use SilverStripe\Forms\GridField\GridFieldConfig_RecordEditor;
use SilverStripe\Versioned\Versioned;
use UndefinedOffset\SortableGridField\Forms\GridFieldSortableRows;
class ImageRasterElement extends BaseElement
{
private static $singular_name = 'Image Raster Element';
private static $plural_name = 'Images Raster Elements';
private static $description = 'Generates an Image Raster for multiple single Elements';
private static $table_name = 'ImageRasterElement';
private static $has_many = [
'SingleElements' => SingleElement::class
];
private static $extensions = [
Versioned::class . '.stagedversioned',
];
public function getType()
{
return 'ImageRasterElement';
}
public function getCMSFields()
{
$fields = parent::getCMSFields();
$singleElements = $this->SingleElements();
$singleElementGridConfig = GridFieldConfig_RecordEditor::create();
$singleElementGridConfig->addComponent(new GridFieldSortableRows('SortOrder'));
$singleElementGrid = GridField::create('SingleElements', 'Single Elements of this Page', $singleElements, $singleElementGridConfig);
$fields->addFieldsToTab('Root.Main', $singleElementGrid);
return $fields;
}
}
This Element has_many single Elements: they look like:
namespace R12page\Elements;
use R12page\Model\News;
use R12page\Model\People;
use R12page\Model\References;
use SilverStripe\ORM\DataObject;
use SilverStripe\Versioned\Versioned;
class SingleElement extends DataObject
{
private static $table_name = 'SingleElement';
private static $db = [
'Reference' => 'Boolean',
'People' => 'Boolean',
'News' => 'Boolean',
'SortOrder' => 'Int'
];
private static $has_one = [
'News' => News::class,
'People' => People::class,
'Reference' => References::class,
'ImageRasterElements' => ImageRasterElement::class
];
private static $summary_fields = [
'News.Headline' => 'News',
'People.Name' => 'People',
'Reference.Headline' => 'Reference'
];
private static $extensions = [
Versioned::class . '.stagedversioned',
];
}
Each Single Element has an has_one to a Data Object. The look like:
namespace R12page\Model;
use R12page\Elements\SingleElement;
use SilverStripe\ORM\DataObject;
use SilverStripe\Versioned\Versioned;
class News extends DataObject
{
private static $table_name = 'News';
private static $db = [
'Headline' => 'Varchar',
'SubHeadline' => 'Varchar',
'Date' => 'Date',
'DatePublished' => 'Date',
'Content' => 'HTMLText',
'IsActiv' => 'Boolean',
'DisplayOnHomePage' => 'Boolean'
];
private static $has_one = [
'Mobile' => Size::class,
'Screen' => Size::Class
];
private static $has_many = [
'SingleElements' => SingleElement::class
];
private static $owns = [
'SingleElements'
];
private static $summary_fields = [
'Headline' => 'Headline',
'Mobile.Title' => 'Mobile',
'Screen.Title' => 'Screen'
];
private static $extensions = [
Versioned::class . '.stagedversioned',
];
}
So far so good. This is what it looks like in the admin Area.
The strange thing is if I add a single Element an try to create it i get this:
When i refresh the page i can see the content of the page and i can save it with out any problems. When if use php_debug everything looks good. I also don't have and error messages in the console which i think, are related to the problem i am having . I just get those to warnings:
But i think the warnings should not be the problem. To be precise i get an error because a font is not loading.
Please help me debug this. I tried the hole day to get this to work. I can not identify the problem i am having.
This is the response i am getting back:
To me it looks okay.
This is the response from the browser networktap:
A view things i can confirm:
Elemental works on a clean install.
If i switch to live mode nothing changes.
If i try to use the content element that ships with elemental it has the same behavior.