Hi, we have the same issue. Managed to make Lightbox load the Youtube videos properly.
1. Enable a Secondary Lightbox
2. Follow all the requirements for video lightboxing
3. In the Secondary Lightbox Settings setting, in the Other tab, confirm what your secondary class name is. My default setting happens to be lbp_secondary.
4. I applied this to my Youtube lightbox, instead of the rel=”lbpModal”, like so:
<a href="https://www.youtube.com/embed/p2GLcFZQd3Y" class="lbp_secondary">
<img src="https://img.youtube.com/vi/p2GLcFZQd3Y/hqdefault.jpg" width="700" height="320"/>
</a>
5. Saved everything, refreshed my browser and it works! ??
Hope this helps.