Hey Sylvio,
I’ve investigated the issue and it seems that the cause of the problem is indeed the fact that the “Background Image/ Source of Slides” metabox does not save its settings properly. Please download this fix I’ve prepared, unzip it, and upload the resulting file into the following folder of your WP installation: /wp-content/themes/Tokyo/includes/supersized/includes/, replacing the existing file. After that please refresh the website and try altering the metabox settings once more. Let me know if this works!
Sincerely,
Andrii / Satori Studio