The problem occurs when you have a Struts 2 convention based web application (WAR) inside an EAR package. And deploy it on jboss. Because of Jboss using the vfszip packaging the open symphony ClassFinder fails with a message like
c.o.x.u.f.ClassFinder - Unable to read URL [vfszip:/.../server/default/deploy/MyEar.ear/MyWebApp.war/WEB-INF/classes/] java.io.FileNotFoundException: /.../server/default/deploy/MyEar.ear/MyWebApp.war/WEB-INF/classes/
So. Nobody solved it in a good way yet. I'm gonna go with this solution, it'll have to do for now.
Update: I tried to implement the linked solution using reflections, but failed. Basically it has the same issue and I'm still left with hard coding the list of classes.