Login impossible after deleting cookies

Login, do stuff, logout, delete cookies => login no longer possible:

14.04.2010 09:42:57 ERROR [org.apache.wicket.RequestCycle.logRuntimeException():1521] Method onFormSubmitted of interface org.apache.wic
ket.markup.html.form.IFormSubmitListener targeted at component [MarkupContainer [Component id = signInForm]] threw an exception
org.apache.wicket.WicketRuntimeException: Method onFormSubmitted of interface org.apache.wicket.markup.html.form.IFormSubmitListener tar
geted at component [MarkupContainer [Component id = signInForm]] threw an exception
at org.apache.wicket.RequestListenerInterface.invoke(RequestListenerInterface.java:193)
at org.apache.wicket.request.target.component.listener.ListenerInterfaceRequestTarget.processEvents(ListenerInterfaceRequestTarg
et.java:73)
at org.apache.wicket.request.AbstractRequestCycleProcessor.processEvents(AbstractRequestCycleProcessor.java:92)
at org.hippoecm.frontend.PluginRequestCycleProcessor.processEvents(PluginRequestCycleProcessor.java:34)
at org.apache.wicket.RequestCycle.processEventsAndRespond(RequestCycle.java:1250)
at org.apache.wicket.RequestCycle.step(RequestCycle.java:1329)
at org.apache.wicket.RequestCycle.steps(RequestCycle.java:1428)
at org.apache.wicket.RequestCycle.request(RequestCycle.java:545)
at org.apache.wicket.protocol.http.WicketFilter.doGet(WicketFilter.java:468)
at org.apache.wicket.protocol.http.WicketFilter.doFilter(WicketFilter.java:301)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.hippoecm.frontend.plugins.login.ConcurrentLoginFilter.doFilter(ConcurrentLoginFilter.java:54)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:852)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:396)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.GeneratedMethodAccessor25.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.apache.wicket.RequestListenerInterface.invoke(RequestListenerInterface.java:182)
... 26 more
Caused by: java.lang.NullPointerException
at org.hippoecm.frontend.plugins.login.RememberMeLoginPlugin$SignInForm$2.load(RememberMeLoginPlugin.java:137)
at org.hippoecm.frontend.plugins.login.RememberMeLoginPlugin$SignInForm$2.load(RememberMeLoginPlugin.java:124)
at org.apache.wicket.model.LoadableDetachableModel.getObject(LoadableDetachableModel.java:122)
at org.hippoecm.frontend.session.UserSession.login(UserSession.java:197)
at org.hippoecm.frontend.plugins.login.RememberMeLoginPlugin$SignInForm.onSubmit(RememberMeLoginPlugin.java:197)
at org.apache.wicket.markup.html.form.Form.delegateSubmit(Form.java:1518)
at org.apache.wicket.markup.html.form.Form.process(Form.java:914)
at org.apache.wicket.markup.html.form.Form.onFormSubmitted(Form.java:876)
... 30 more