Then it would be the Widgetkit one. A couple things…I noticed the site is using version 2.3.3. which you may want to consider upgrading to the recent one of Celestial Reloaded 2.4.1. Also, double check that the widget that was loading the slideshow is still in the widget position it was assigned to.
I just setup a test WordPess locally, installed Celestial Reloaded, and then put in the widgetkit (version 1.4.7) plugin, created a couple slides, and it appears to be working.
WP 3.8 is quite different and sometimes upgrading a major WP update can do some funky things. But from what I can see on my test site, the widgetkit plugin is compatible with WP 3.8.