Hello,
I use your Url to process some tests directly on your website, and it's impossible to reproduce your issue...
But I have some theory thanks to your screenshots.
It seems that you have, the popup add to cart update
So, I guess that the cart is updated BUT I think that the error just unable your cart module to be updated (that must be an Javascript error I guess)
But impossible to say more as we are unable to reproduce on your website.
Also note that your Cart module isn't the default HikaShop cart :
I don't understand your "double" html cart structure...
Hope this will help you.
Regards