If you look at the source and see that your <c:url> tag is still there, it means that you've got a setup issue that's causing the tag to not be interpreted correctly.
It has nothing to do with mixing JavaScript and JSTL.
I have solved the problem. I forgot to insert <%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %> into the JSP, and I have thought it was a JavaScript-problem.
You see, I am a really experienced JSP-developer. (just kidding)
Ben & Bear, thank you for the quick help. You pointed me in the right direction.