Apple event handler failed error suddenly thrown by unchanged script

I had a working AppleScript running as a rule in Mail, and suddenly it stopped working. I'm not sure exactly what changed when it stopped, as I didn't notice for a few weeks, but it may have been an update to Mac OS X.

Here is the original script, which was working perfectly before:
http://discussions.apple.com/message.jspa?messageID=7818650

Now when it gets to the following line, it throws an error "Apple event handler failed".
set TheDetails to content of ThisOne

Note that the script hasn't changed at all, and neither has the content of the emails that it parses. It just simply stopped working.

I really don't have an idea where to start with this. Any help/suggestions very welcome.