User Service problem

Whenever I try to use User Service plugin, I get the following exception:

HTTP ERROR: 500

gnu/inet/encoding/Stringprep

RequestURI=/plugins/userService/userservice

Caused by:

java.lang.NoClassDefFoundError: gnu/inet/encoding/Stringprep
     at org.jivesoftware.openfire.plugin.userService.UserServiceServlet.doGet(UserServiceServlet.java:130)
     at javax.servlet.http.HttpServlet.service(HttpServlet.java:707)
     at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
     at org.jivesoftware.openfire.container.PluginServlet.handleServlet(PluginServlet.java:251)
     at org.jivesoftware.openfire.container.PluginServlet.service(PluginServlet.java:91)
     at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
     at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:487)
     at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1093)
     at org.jivesoftware.admin.PluginFilter.doFilter(PluginFilter.java:70)
     at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1084)
     at org.jivesoftware.admin.AuthCheckFilter.doFilter(AuthCheckFilter.java:146)
     at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1084)
     at com.opensymphony.module.sitemesh.filter.PageFilter.parsePage(PageFilter.java:118)
     at com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:52)
     at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1084)
     at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:360)
     at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
     at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:181)
     at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:726)
     at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:405)
     at org.mortbay.jetty.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:206)
     at org.mortbay.jetty.handler.HandlerCollection.handle(HandlerCollection.java:114)
     at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
     at org.mortbay.jetty.Server.handle(Server.java:324)
     at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:505)
     at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:829)
     at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:514)
     at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:211)
     at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:380)
     at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:395)
     at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:488)

I have checked that the plugin is enabled, and I pass the secret pass correctly. I'm using Linux Mint, and libidn seems to be installed. Any idea about what may be causing this problem?

Regards