Hello, @Joan,
First of all, please deactivate all the plugin on your site, and then go to Appearance >> Themes and Activate the theme from there, after that Install the XStore Core plugin from XStore >> Dashboard >> Plugin Installer and I am sure you will not face this issue anymore.
Now after that you have to activate your site plugins again one by one and check which one is generating the issue on your site.
Regards 8Themes Team.