You deactivate the group "TreeGeneration" and "Start" from your Scene1_Event event sheet and do not reactivate them in your "On Start of layout" event.
So the second time you come to this layout, no trees are generated and you cannot Start the layout.
Moreover, be sure to reset the global variables before you direct back to the Scene1 layout (directly in the Scene2_Event and Scene3_Event event sheets before the "Go to Scene1" action).
Moderation note: please refrain from posting the same post over multiple forums in the future.