Hmmm.. I hadn't even thought about the Itch app itself. I'll try and see what I can do. I'll make a note on the game page for now, saying that something in the app browser isn't happy with the save code.
The code snippet I'm using for the save function is from an IntFiction post by the developer of SugarCube. But with browsers being what they are it can be quirky to support all different iterations of it.
Thanks for letting me know!