I have been running this plugin for quite a while on a site and recently updated the plugin. Then today I went to update the ticker deadline and was unable to access anything in my Widgets Admin area. I couldn't select a drop down menu or drag and drop items -- basically the page would load but I couldn't do anything.
The countdown ticker was the first Widget, so I tried disabling the plugin and everything worked again.
I'm running WordPress 3.5.1 and Version 2.2.9 of the T(-)Countdown plugin.