[xwiki-users] HTTP Status 500 @ Register-Page on fresh installed xwiki 5.2

Hey guys, i run into an exception on my newly installed xwiki after klicking the register-button: HTTP Status 500 - _____ type Exception report message description The server encountered an internal error () that prevented it from fulfilling this request. exception javax.servlet.ServletException: Servlet execution threw an exception com.xpn.xwiki.web.ActionFilter.doFilter(ActionFilter.java:121) org.xwiki.wysiwyg.server.filter.ConversionFilter.doFilter(ConversionFilter.j ava:144) com.xpn.xwiki.plugin.webdav.XWikiDavFilter.doFilter(XWikiDavFilter.java:66) org.xwiki.container.servlet.filters.internal.SavedRequestRestorerFilter.doFi lter(SavedRequestRestorerFilter.java:208) org.xwiki.container.servlet.filters.internal.SetCharacterEncodingFilter.doFi lter(SetCharacterEncodingFilter.java:111) root cause java.lang.NoClassDefFoundError: Could not initialize class com.xpn.xwiki.web.RegisterAction sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAcces sorImpl.java:57) sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstruc torAccessorImpl.java:45) java.lang.reflect.Constructor.newInstance(Constructor.java:532) java.lang.Class.newInstance0(Class.java:374) java.lang.Class.newInstance(Class.java:327) org.apache.struts.util.RequestUtils.applicationInstance(RequestUtils.java:14 3) org.apache.struts.action.RequestProcessor.processActionCreate(RequestProcess or.java:292) org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:230) org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196) org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414) javax.servlet.http.HttpServlet.service(HttpServlet.java:617) javax.servlet.http.HttpServlet.service(HttpServlet.java:717) com.xpn.xwiki.web.ActionFilter.doFilter(ActionFilter.java:121) org.xwiki.wysiwyg.server.filter.ConversionFilter.doFilter(ConversionFilter.j ava:144) com.xpn.xwiki.plugin.webdav.XWikiDavFilter.doFilter(XWikiDavFilter.java:66) org.xwiki.container.servlet.filters.internal.SavedRequestRestorerFilter.doFi lter(SavedRequestRestorerFilter.java:208) org.xwiki.container.servlet.filters.internal.SetCharacterEncodingFilter.doFi lter(SetCharacterEncodingFilter.java:111) note The full stack trace of the root cause is available in the Apache Tomcat/6.0.35 logs. Do you have any idea why? I installed it via aptitude on my ubuntu 12.04 x64. Regards, Eric