It looks like there is something that messes directly with Unchecky service and the way it operates against the kernel-mode subsystem (in this case: winlogon.exe+ad15).
What antivirus software do you use?
Have you ever check the system against rootkits or other malware? There is a possibility for it to intercept Unchecky service calls and mess with them, so you might get this error. I suggest running a GMER scan.

And so yes, the first stable version has been released (:
https://code.google.com/p/auto-checkbox-unchecker
if you can, please list any websites that bundle malware/crapware, the filters'll be set by default (:

I've recently started work for an cross-browser checkbox unchecker, which would be good in dealing with these situations. At the moment, my target is Chrome, but it'll work (probably) in Firefox and Opera without scratch (since it's JS and it operates by DOM).
....and since my script will be BSD-licensed, there's no problem to use my code in your projects (: