Hello,
3/ When you face the problem and then click reload the page, it is the same?
Maybe after solving the error 500 from server-side it will be solved also.
Meanwhile, try to disable all the third-party plugins (including cache plugins), that do 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 a reason for the problem.
Regards