Hi! I have the same trouble. I can't save product when I try to add more than one categories through associative categories Anybody help? Here is the code with error from AJAX requests.
This value is not valid. tree
Symfony\Component\Validator\ConstraintViolation
Object(Symfony\Component\Form\Form).children[step1].children[categories].children[tree] = [0 => 2, 1 => 40]
Caused by:
Symfony\Component\Form\Exception\TransformationFailedException
The choices "40" do not exist in the choice list.