I've encountered a few mods in the recent past that bug out and finger the UIDropDownMenu. It can be either the mod not handling it properly (great guide on how to use it properly here) or something else unrelated in a mod triggering it.
You've got several very old versions of mods there (your version of AtlasLoot--which was updated last week--dates from a year ago, as does your Auctioneer, etc.). Update everything and try again.
The error posted in this thread is over a year old, and any possible solutions offered in response overlooked that fact and are therefore not at all relevant if you are getting a similar error with up-to-date addons here in 2009. If you are getting a similar error, start a new thread. Thread locked.