Impossible to add gift cards

Hi,
I’ve been using this plugin with another web site in the past, with success.

But with this web site, I can not manage to add a gift card to the basket, I get the message “Your cart is currently empty”.
Strange thing, if I set the gift card in the wish list, I can then add it to the cart from the wish list !

The page I need help with: [log in to see the link]