Adding to cart produces the message "An error occurred" in the pop up window

Hi,

I don't see why it would do that.
And since the problem happens and the product is still added to the cart, and that the popup content is still displaying properly ( you can check that here: fricanospizzaalpine.com/order-online/che...otice/tmpl-component ), I would say that it might be linked to the template or to the encoding of the server, or some optimization extension.
The first thing I would recommend is to switch to the default template of Joomla temporarily and see if it works with it.
Also, as it might be linked to a plugin, you could try deactivating the hikashop and system plugins one by one in the Joomla plugins manager.
And finally, as it looks like it is linked to the popup display, you could try changing the "After a product is added to the cart" setting of the HikaShop configuration to not have a popup after the add to cart process in order to circumvent the issue.