I had a look and changed the password for the test admin account (to prevent abuse!). Please delete that account after this... And check for any unwarranted changes on your site or theme or plugin files or any strange files uploaded to wp-content/uploads/ or wp-content/plugins/ ... just to be sure. Seriously, your site can be infected in minutes, without you noticing anything.
Anyway, this is my conclusion: your theme has FancyBox integrated. This conflicts with Easy FancyBox.
The solution is to either :
A. use the themes own possibility to display youtube movies by giving the link
class="et_video_lightbox" (I did that on the test page) and dump the Easy FancyBox plugin... or ...
B. edit your theme to strip out all the FancyBox related stuff from the theme and continue using the Easy FancyBox plugin (in un-edited version)
I'd guess option A would be the easiest but if you need help with option B, you can hire me via http://4visions.nl/en/contact/ :)