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Hi there,
The function wcusage_applied_coupon_check_allow_customer which is attached to the hook woocommerce_update_cart_action_cart_updated should return a changed boolean value or the original value passed into the function, otherwise other plugins which depend on this hook might not work as expected.
That is the case for our plugin Fluid Checkout PRO, which expects a boolean value after running that hook. If no value is returned by the hook, our plugin will consider it an error and will not update cart item quantities on the cart page.
We have already fixed this issue on our plugin, however, I thought you might be interested in fixing it on your side as well.
Best,
Diego.