Hey @rotation,
I checked your site using macOS Catalina with Safari 13 and the builder is loading fine. In the secure note is a screenshot of your home page opened in the Content Builder.
If you also remember in your other thread, I created a test header and page in the same site you’re having this issue.
You probably have something on your computer conflicting with Pro most probably a browser extension. If you’re using browser extensions, please deactivate all of them and test Pro again. Please also test in incognito mode and other browsers.
Another thing you can try, if applicable, is open Pro in another Apple device and also a different internet connection.
Thanks.