Hi,
You have a strange issue on your website.
If I try to do the procedure with the debug mode of Joomla activated, I get this call stack for the URL called by the button of the popup:
monosnap.com/file/ZDuy6yZyj98IGqAQGGJjedQum3oMjN
As you can see the URL called has option=com_hikashop which indicate to Joomla that HikaShop is to be called to handle the request.
But if you look on the right side at the call stack, you can see that no HikaShop file is called by Joomla.
This indicates that either the server, the htaccess, a system plugin or a core file of Joomla has modified the request resulting in the "option" parameter being missing when Joomla is deciding which component to call. It thus defaults to Joomla's com_content, which displays that error as the parameters are not correct for it.
I wanted to do some more testing by doing some view overrides, but when I saved my change to a view file in Display>Views, I actually got the same 404 error.
So that indicates that the issue you have comes from a security mechanism you have in place on your server/website which detects something suspicious (like the HTML of an email, or the PHP of a view file) and clear all the parameters sent to the website by the browser.
I'm afraid you won't be able to use that feature unless you configure or disable that security mechanism so that it doesn't interfere with the normal functions of HikaShop.
