http:// vs. https://

What tool/website are you using to do your SEO checks? I just loaded one of many sites I've created with Nicepage and got no warning from the browser regarding mixed content, despite verifying that my generated code also contains the http references you describe above. I want to see if my site generates the same SEO alerts you're seeing.

It happened when I tried to register my websites with Bing. You may not see it with Google but the problem does indeed not have anything to do with the way it is discovered, it is real. I have checked the files produced by the nicepage editor, and there are indeed certain links inserted by the editor that are HTTP:// instead of HTTPS://. The rule for a safe website is not only that it uses the https protocol, but that there should be no links to insecure sources, and there is no reason why the editor need to create HTTP:// links when these websites can be accessed with HTTPS://.
Your site certainly has the same links. Open the HTML file of any page with Atom and search for "HTTP://"

To the developers, please fix this issue ASAP. All connections should be secured and encrypted connections!

Nothing justifies a http:// connection in this day and age.