Hi p1erstef,
It sounds like your browser is blocking secure content embedded in a non secure web page. This is done because this kind of thing is sometimes used in so-called man-in-the-middle attacks to steal private data. You are not using it that way (I suppose) but the browser simply blocks this kind of activity to prevent exposing the user to security risks.
You might be able to set your browsers security level more relaxed but that will not solve the issue for other visitors.
Embed a simple iframe (not a link that opens in FancyBox) in your page content with the same URL. Then browse inside that iframe to the ssl page and you should get the same result… If not, then please share a link to your site ??