My first start of hudson worked well. Now it crashed after a weekend of idle time and I can't restart it.

calling the web interface, I get this trace

07:51:14,952 ERROR [[Stapler]] Servlet.service() for servlet Stapler threw exception
java.lang.ClassCastException: org.kohsuke.stapler.jsp.JSPFacet cannot be cast to org.kohsuke.stapler.Facet
at org.kohsuke.stapler.MetaClass.buildDispatchers(MetaClass.java:85)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:50)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:118)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:47)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:118)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:47)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:118)
at org.kohsuke.stapler.MetaClass.<init>(MetaClass.java:47)
at org.kohsuke.stapler.WebApp.getMetaClass(WebApp.java:118)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:492)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:441)
at org.kohsuke.stapler.Stapler.service(Stapler.java:123)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:94)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:86)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:47)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:164)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(SecurityAssociationValve.java:182)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:432)
at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:84)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.jboss.web.tomcat.service.jca.CachedConnectionValve.invoke(CachedConnectionValve.java:157)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:262)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:446)
at java.lang.Thread.run(Thread.java:619)