Like many others here, I was having problems with Easy Fancybox after doing the 1.5.0 update. My problem was that clicking on images did not bring them up in the lightbox, but did load them at the bottom left of the page.
I've spent 2 days researching the problem, and have done everything suggested including shutting off all of my plugins except Easy Fancybox and I changed themes over and over again.
I have a theory that the problem might relate to the "deflation" of the css file done at the beginning of the easy-fancybox.css.php file and how that is handled on certain servers - especially shared hosting platforms where there isn't much enduser control.
The only working solution I have found is to make a change to the easy-fancybox-class.php file, line 469 (at or about).
wp_enqueue_style('easy-fancybox-css', EASY_FANCYBOX_PLUGINURL.'easy-fancybox.css.php', false, FANCYBOX_VERSION, 'screen');
wp_enqueue_style('easy-fancybox-css', EASY_FANCYBOX_PLUGINURL.'fancybox/jquery.fancybox-1.3.5.css', false, FANCYBOX_VERSION, 'screen');
Doing that brings the jquery.fancybox css file into the header, without any deflation.
The only problem with doing that is subsequent updates will destroy the modification. It also doesn't solve the problem, it just puts a bandaid on it.
I would be curious to know what the php requirements would be for this plugin? I can't seem to locate any information regarding that and it might help me fix the problem for all sites I need to update.