set_overlay_tile_server creates empty views

[Reblaus73]

Dear all, following the example codes, it appears that the overly of tiles is not working.
When calling set_overlay_tile_server the output map is empty.
The tile-server is confirmed, because replacing 'set_overlay_tile_server' in the code here below to 'set_tile_server' shows images, but without the underlying main map.

How could this be made work? - thanks

I reduced the code to the strictly necessary:

import tkinter
from tkintermapview import TkinterMapView
root_tk = tkinter.Tk()
map_widget = TkinterMapView(root_tk)
map_widget.pack(fill="both", expand=True)

map_widget.set_overlay_tile_server("http://tiles.openseamap.org/seamark//{z}/{x}/{y}.png")

map_widget.set_position(52.96595, 4.78467)
root_tk.mainloop()