Hello,
Try to disable “AJAX Add To Cart For Simple And Variable Products” via Theme Options and check how it works then.
Please try to disable all the third-party plugins (including cache plugins), that does not come bundled with XStore https://xstore.helpscoutdocs.com/article/36-included-plugins , clear the cache of the browser, and check then. Maybe one of the additional plugins is the reason for the problems.
Let us know the results.
Regards