Hello,
here one exception from the error.log:
2012.02.09 18:03:38 org.jivesoftware.openfire.container.PluginManager - Error loading plugin: C:\Documents and Settings\a126380\Desktop\openfire2\plugins\clustering
java.lang.NoSuchMethodError: com.tangosol.net.CacheFactory.getInvocationService(Ljava/lang/String;)Lcom/tangosol/net/InvocationService;
at com.jivesoftware.util.cache.CoherenceClusteredCacheFactory.startCluster(CoherenceClusteredCacheFactory.java:96)
at org.jivesoftware.util.cache.CacheFactory.startClustering(CacheFactory.java:580)
at org.jivesoftware.openfire.cluster.ClusterManager.startup(ClusterManager.java:270)
at com.jivesoftware.openfire.ClusteringPlugin.initializePlugin(ClusteringPlugin.java:133)
at org.jivesoftware.openfire.container.PluginManager.loadPlugin(PluginManager.java:483)
at org.jivesoftware.openfire.container.PluginManager.access$300(PluginManager.java:80)
at org.jivesoftware.openfire.container.PluginManager$PluginMonitor.run(PluginManager.java:1067)
at org.jivesoftware.openfire.container.PluginManager.installPlugin(PluginManager.java:195)
at org.jivesoftware.openfire.admin.plugin_002dadmin_jsp._jspService(plugin_002dadmin_jsp.java:168)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:530)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1216)
at com.opensymphony.module.sitemesh.filter.PageFilter.parsePage(PageFilter.java:118)
at com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:52)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1187)
at org.jivesoftware.util.LocaleFilter.doFilter(LocaleFilter.java:74)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1187)
at org.jivesoftware.util.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:50)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1187)
at org.jivesoftware.admin.PluginFilter.doFilter(PluginFilter.java:78)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1187)
at org.jivesoftware.admin.AuthCheckFilter.doFilter(AuthCheckFilter.java:164)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1187)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:425)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:119)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:494)
at org.eclipse.jetty.server.session.SessionHandler.handle(SessionHandler.java:182)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:933)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:362)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:867)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:117)
at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:245)
at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:126)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:113)
at org.eclipse.jetty.server.Server.handle(Server.java:334)
at org.eclipse.jetty.server.HttpConnection.handleRequest(HttpConnection.java:559)
at org.eclipse.jetty.server.HttpConnection$RequestHandler.content(HttpConnection.java:1007)
at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:747)
at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:203)
at org.eclipse.jetty.server.HttpConnection.handle(HttpConnection.java:406)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:462)
at org.eclipse.jetty.util.thread.QueuedThreadPool$2.run(QueuedThreadPool.java:436)
at java.lang.Thread.run(Unknown Source)