The net-site has ssl certificate. Any http page is rerouted towards the same, but https page (otherwise https) by .htaccess. Everything works fine, but 404.shtml will get a burglar warning "This web page consists of content that won't be shipped utilizing a secure HTTPS connection..." in IE. Exactly the same behavior is within every other browser. How you can exclude that error?

404.shtml web-page was produced using web-hosting user interface wizard. The file 404.shtml was produced instantly.

Possibly the instantly produced file includes links, images or any other assets fetched via http. If you're able to change these to https links, you need to steer clear of the problem. There's afaik not one other method to result in the browser not warn relating to this, a minimum of inside a mix browser way.