Why am I getting this weird 'Mixed Content' Error?

Question

I am currently optimizing my website a little bit using, among other tools, PageSpeed Insights. In the "Best Practices" section i stumbled over a load of mixed content errors. The source on my website always regards embedded IFrames and looks as follows:

homeimg02/description02.html:1:0(examplesite.de)

It also contains the following description:

Mixed Content: The page at 'https://examplesite.de/' was loaded over HTTPS, but requested an insecure font 'http://www.formerexamplesite.de/404page.html'. This request has been blocked; the content must be served over HTTPS.

I have no idea from where my page even pulls the old URL. To clarify, "formerexamplesite.de" was the url i used while writing and testing the site, but it now sits on "examplesite.de" and the old domain name appears nowhere in my code, i checked multiple times.

The "description02.html" is an embedded IFrame. There are no differing fonts used or loaded in that html document.

The fonts used are saved on the server and loaded in the style tag:

/* montserrat-600 - latin */
        @font-face {
                font-family: 'Montserrat';
                font-style: normal;
                font-weight: 600;
                font-display: swap;
                src: local(''),
                    url('resources/fonts/montserrat-v25-latin-600.woff2') format('woff2'),
                    url('resources/fonts/montserrat-v25-latin-600.woff') format('woff');
                }

There are a couple dozen Errors like this, regarding different IFrames. I know using IFrames to embed stuff is not a good solution, but it was necessary in my situation, so other people could alter content on the webpage without touching the main document.

I'd like to understand what this error even tries to tell me and how I can fix it, if there is an error in my code causing this.

EDIT: This comes up, when checking the browser requests.

Request URL:
https://examplesite.de/resources/homescreen/homeimg03/resources/fonts/montserrat-v25-latin-600.woff

I have the suspicion, that my pathing in the IFrame requesting the font might be wrong, as its set to local, thus creating this weird mess of a path. This still does not explain though, how the website gets my former domain name.