Today I solved the problem that started 2 days ago. Here's an exerpt from http://husted.com/struts/FAQ/installation.htm which was the solution:
* For reasons discussed further below, "struts.jar" MUST NOT be on your system CLASSPATH when you start your servlet container.
* When a loaded class references other classes, it looks in its own class loader first, and then up the hierarchy. Under NO circumstances will it go down the hierarchy.
* Therefore, if a Struts class is loaded from the system class path, that class CANNOT see any of your application classes in WEB-INF/classes or WEB-INF/lib -- you will get ClassNotFoundException instead.