Cart isn't working properly

Hi,
Firstly, thanks for your awesome theme! I’ve purchased a few times now!

We’re having issues with the basket on one of the sites. It used to work ok up until about 2 or 3 weeks ago.
It’s doing all sorts of strange things. If you add something to the cart and view the cart and try to delete it then click “back to shop” it takes you back to the basket, also it won’t let me change quantites. https://fatstickboards.com/
Strange thing is it seems to work ok if I’m logged in. But as soon as I’m logged out it stops working.

I’ve had a friend look at it and he says this error is happening:
‘PHP message: The The “add_to_cart_fragments” hook uses out of date data structures and function is deprecated since version 3.1.0. Replace with woocommerce_add_to_cart_fragments’

Thanks

Ticket