I have made a scene according to what i can see on your picture. It worked fine. here is the code:
extends Control
onready var titel_node = $WindowFrame/Titel
func _ready():
set_title("text")
func set_title(t):
var titel = t
if titel_node == null:
titel_node = $WindowFrame/WindowFrame/Titel
titel_node.set_text(titel)
Does this work? If not, consider making the scene again