Hi,
I don't think this has anything to do with your template.
With the backend access you provided, I temporarily switched to the default template of Joomla, and I made the same test, and got the same result with the 404 error.
In fact, the issue might actually not be in your website at all because the error page comes from Nginx, your web server, and not from your website.
So it's not that Joomla's router is not able to route the URL, but that Nginx directly blocks the URL for some reason.
I would recommend looking for the URL in the access log and / or error log of nginx on your server to see what it has to say about it.
Maybe there is some custom rule catching on the "confirm" word in the URL which triggers nginx into generating a 404 error ?