Hi Olga, I had other sites and never experienced such an error. It must have something to do with the integration of xstore, like the support of WP Rocket mentioned…
“The way that the theme is displaying the “added to cart” message is not fully AJAX-ed, so it gets cached.
If you are not able to modify the theme to solve that, or if the theme developer can’t provide a solution, I think you can resolve it either by:
redirecting to the cart page after addition,
or by NOT enabling the AJAX add to cart buttons on archives.
Cheers!”
But I think that are very bad solutions: redirect to cart page or deactivating ajax add to cart…
Hope you can sort that out…