Well if you're always using the same copy of jmeter and starting it the same way, I don't think it would be a missing plugin. It also doesn't sound like you're using the include controller at all, so that shouldn't be a problem.
You said earlier that you had the test (JMX file?) open in wordpad. Is that something you do often? Are you changing and saving it with wordpad? Perhaps that's what's corrupting the XML. -- Bruce Ide flyingrhenqu...@gmail.com