Hello, Manos
1/ Please try: Theme Options > Speed Optimization > Always Load Wc-Cart-Fragments > On.
2/ 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 problem.
Best Regards,
8Themes Team