I don't think your "404.html" page is what you intended to link :P
404.html will be the one not displaying a 404 for some reason, but 404.htm does lol.
You are using rails, remove the default 404.html from your public folder and it will stop serving the default rails 404.html.
- https://serpapi.com/404.htm
- https://serpapi.com/500.html