Hi,
It turns out that it's an issue following another improvement we made last year to allow add to cart tags to use the cart notification popups even on pages where HikaShop is not loaded. That system would load the plugin without checking whether it is enabled or not.
I've made a patch in order to fix this and added it to the website for which you provided the access. I can confirm that it fixes the problem.
For anyone else with the issue, I've also added the patch in HikaShop. So you can download the install package of the latest version on our website and install it on yours in order to get the patch.