The problem is as following
When setting a rule :
$crud->setRule('voornaam', 'lengthMax', '100');
$crud->setRule('voornaam', 'required');
And using readOnlyEditFields :
$crud->readOnlyEditFields(['voornaam']);
I cant save a form that I edit, because it says it has no length and is required. But the field is in fact filled, but not editable.