It could also be the way the moov atom(header info) is placed in the video. If someone has converted the video some of the free converters put the header at the end of the video instead of at the top. The youtube upload many times is smart enough to find it and move it to the top if it is not there. But it is not a perfect science.
What i have done to my video is placed a default image in there so that if it does not load a thumbnail then the default image will be the thumbnail, is better than looking at a black box.
special coding mod, and even with that it is not 100 percent full proof. I still get black boxes at times just not as much as before, there might be 2 out of 12 on a page that is a black box, the others are either def image or thumbnail.
<iframe width="420" height="315" src="https://www.youtube.com/embed/8fXevbTDOQQ" frameborder="0" allowfullscreen="allowfullscreen"></iframe>
And yes, just recently I saw where the embed code has changed as the checkbox for ssl no longer exist. Either way.. no image... sigh.
did anyone managed to solve it care to share the fix.
I hope a fix will come soon.