The error is only happening in some browsers. it works fine on Chrome, but doesn't seem to be working on Miscorsoft Edge, and possibly Safari and Firefox.
The issue is that once the cart has been updated once, subsequent attempts to add/remove products are unsuccessful. Some customers are also reporting that they can't update the cart at all even the first time around trying to add a product, although it says that the product was added. But when you go to the cart it's empty. I could not reproduce that issue myself, however i can reproduce the updating issue.
The php_error log only contains the following error:
The WC_Coupon::exclude_sale_items function is deprecated since version 3.0. Replace with WC_Coupon::get_exclude_sale_items