Youtube iframe in lightbox randomly shows a black screen

I'm experiencing a weird problem with youtube iframe embedding. A button triggers a lightbox with an iframe but, especially on iOS Safari (but not only), it only shows a black screen. Iframe is correctly rendered in DOM but nothing happens. If I load the iframe URL directly and then go back and click again the button, the iframe shows fine.

You can reproduce that on http://ift.tt/1FUhgim and scroll down to any "watch video" button.

The app is made with Ember and the lightbox is managed by Liquid-Fire.

I have no clues...