just removed the complete directory and rerun the build task ..
this kind of problem, better search at google.
http://archives.real-time.com/pipermail/cactus-user/2003-September/003477.html
http://groups.google.co.uk/groups?q=java.lang.ClassCastException%3A%20com.octetstring.vde.backend.BackendRoot&hl=en&lr=&ie=UTF-8&sa=N&tab=wg
