One possible cause is that you have some jars -- e.g. struts.jar -- in a shared lib directory such as server/lib or common/lib. If so, this is a bad idea as you're setting yourself up for classloader nightmares. The problem is that classes in your web app can see classes in these shared directories, but this doesn't work not the other way around: a class loaded from server/lib or common/lib cannot instantiate any classes that are only available in your web application (i.e. WEB-INF/classes or WEB-INF/lib).
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