Hi all is fixed. Cause of the error was because Storefront template is not the default template and your default template needed few extra files, such as cart.php and json.php.
Only problem remains is language file in nl-NL folder. Please copy and rename en-GB.tpl_gk_storefront.ini to nl-NL.tpl_gk_storefront.ini and upload it to nl-NL language folder.
Let me know if you need further help. See you around...