I don't know if this would answer your problem, but if your app server has JSTL examples, use the JAR files and URI from the examples in your application. For example, I had the same error that you did when I was working with Tomcat. I had inadvertently used the wrong JAR directory (standard-1.0/lib rather than standard/lib ) when referencing the Jakarta JSTL libraries.
Don't know why a re-start would solve the issue, but at least you're off and running !!