Ok - sorry for being a noob here - but i still can't see what i'm doing wrong:
In the "Advanced iframe plugin" it says:
Enter the full URL to your page. e.g. http://www.site.com. You can also add parameters to this url like http://www.site.com/test.php?iframe=true. then you can check this variable and use it to e.g. hide some elements in the iframe.
So I copy paste this url and you say it's my iframe source. I can't use the aforementioned embed code right?
It's works fine with a couple of other sites i tested by the way!