When I open your demo page at:
vmuikit.joomlapro.com/
and click the "Add to Cart" button for the "Focusrite Scarlett 2i2 USB-lydkort" product, I get a modal saying, "1 x Focusrite Scarlett 2i2 USB-lydkort was added to your cart."
However, after closing the modal, when I go to the top of the page and click the cart module next to the search module, the cart appears empty, even though the cart total is displayed on the button. After refreshing the page, the cart functions correctly.
I’m experiencing a similar issue on another development site of mine, but it occurs on the product details page.
We have tested the reported issue on our demo site and did not find any problems. The cart modal is working as expected may be its a browser specific one.