ClassCastException when clicking external link button in xinha

• log in to cms
• go to documents
• browse to a document (with a xinha field)
• open document
• in html field, select some text and click on the external link button
• nothing happens, on the console you get a ClassCastException:

14.01.2009 16:48:42 ERROR [org.apache.wicket.RequestCycle.logRuntimeException():1437] org.apache.wicket.model.Model cannot be cast to org.hippoecm.frontend.plugins.xinha.services.links.ExternalXinhaLink
java.lang.ClassCastException: org.apache.wicket.model.Model cannot be cast to org.hippoecm.frontend.plugins.xinha.services.links.ExternalXinhaLink
at org.hippoecm.frontend.plugins.xinha.dialog.links.ExternalLinkDialog.getLink(ExternalLinkDialog.java:117)
at org.hippoecm.frontend.plugins.xinha.dialog.links.ExternalLinkDialog.<init>(ExternalLinkDialog.java:47)
at org.hippoecm.frontend.plugins.xinha.dialog.links.ExternalLinkBehavior.respond(ExternalLinkBehavior.java:42)
at org.apache.wicket.ajax.AbstractDefaultAjaxBehavior.onRequest(AbstractDefaultAjaxBehavior.java:297)
at org.apache.wicket.request.target.component.listener.BehaviorRequestTarget.processEvents(BehaviorRequestTarget.java:100)
at org.apache.wicket.request.AbstractRequestCycleProcessor.processEvents(AbstractRequestCycleProcessor.java:91)
at org.apache.wicket.RequestCycle.processEventsAndRespond(RequestCycle.java:1175)
at org.apache.wicket.RequestCycle.step(RequestCycle.java:1252)
at org.apache.wicket.RequestCycle.steps(RequestCycle.java:1353)
at org.apache.wicket.RequestCycle.request(RequestCycle.java:493)
at org.apache.wicket.protocol.http.WicketFilter.doGet(WicketFilter.java:355)
at org.apache.wicket.protocol.http.WicketFilter.doFilter(WicketFilter.java:200)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1084)
at org.hippoecm.frontend.plugins.login.ConcurrentLoginFilter.doFilter(ConcurrentLoginFilter.java:54)
at org.mortbay.jetty.servlet.ServletHandler$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:722)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:404)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:139)
at org.mortbay.jetty.Server.handle(Server.java:324)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:505)
at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:828)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:514)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:211)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:380)
at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:395)
at org.mortbay.thread.BoundedThreadPool$PoolThread.run(BoundedThreadPool.java:450)