Cart not accepting quantity

Using the latest version – the cart is acting weird.

When I put in more the two items it returns to having just one item in the cart.

Please try it at: https://julieoestergaard.dk/butik/

I have Wordfence installed – are there any known conflicts between them maybe?

THANKS!

https://www.ads-software.com/plugins/wordpress-simple-paypal-shopping-cart/