Antonson,As per your attached code... there is no successor element which is parent element of successorset element.Put the same in all of your tasks. Hope that would work fine.
That's not entirely right - with respect to the submittask, they do not have a successors element - only a successorset -or- a success and optional failure element.However, you are right with respect to the externaltask - as it requires a successors container around the successorset elements.