Yes, embedding HTTP in an HTTPS site will break it. Some browsers simply won't display a secure page that has insecure content (images, fonts, embedded sites, etc).
The next problem is that STS has told your browser that it should ONLY get that site over HTTPS, until max-age has expired (six months). And you've disabled HTTPS, so your browser can't see the site at all, including any headers.
I think you'll need to re-enable HTTPS, with max-age=0. Then visit the site again.
So, max-age=0 won't help if SSL is broken (for example, your certificate has expired and you're having trouble renewing it).
Something else you could do is to use a different browser that's never seen the STS. But that's obviously not a good solution, except as a last resort.