I have had similar behavior with my pure entity units when the subscene is enabled. I forget what the error I get is though. If your subscene is enabled when you test, try disabling it.
One other thing to note, AstarPath cannot use anything in a subscene to bake the graph. So you need to bake it in the editor with the subscene enabled, then turn off AstarPath bake on start (or whatever that setting is called) or else the editor bake gets overwritten when the game starts.