Have you installed a new version of Internet Explorer lately? All references I find for this error are IE related. And BitComet has an integrated version of IE.
I have also encountered this error, but only in the last 6-8 months or so. It happens to some particular torrent, can only be cured by deleting the download and starting over, and often recurs when it does happen, at which point I usually give up.
The fatal x message “the requested lookup key was not found” coninues on. There must be some solution to this as I was using version 1.37 64bit with windows 8.1, downloading up to April 16, 2014, and then…the Red X.