Can any one explain what this error means please?

2008-08-25 14:48:50,971 ERROR [btpool0-10] [] log - Nested in org.apache.jasper.JasperException: java.lang.NullPointerException:
java.lang.NullPointerException
at com.zimbra.webClient.servlet.SkinResources.parseUs erAgent(SkinResources.java:658)
at com.zimbra.webClient.servlet.SkinResources.doGet(S kinResources.java:154)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:707)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:820)
at org.mortbay.jetty.servlet.ServletHolder.handle(Ser vletHolder.java:487)
at org.mortbay.jetty.servlet.ServletHandler.handle(Se rvletHandler.java:362)
at org.mortbay.jetty.security.SecurityHandler.handle( SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(Se ssionHandler.java:181)
at org.mortbay.jetty.handler.ContextHandler.handle(Co ntextHandler.java:716)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebA ppContext.java:406)
at org.mortbay.jetty.servlet.Dispatcher.include(Dispa tcher.java:192)
at org.apache.jasper.runtime.JspRuntimeLibrary.includ e(JspRuntimeLibrary.java:997)
at org.apache.jsp.public_.login_jsp._jspService(login _jsp.java:557)
at org.apache.jasper.runtime.HttpJspBase.service(Http JspBase.java:93)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:820)
at org.apache.jasper.servlet.JspServletWrapper.servic e(JspServletWrapper.java:373)
at org.apache.jasper.servlet.JspServlet.serviceJspFil e(JspServlet.java:477)
at org.apache.jasper.servlet.JspServlet.service(JspSe rvlet.java:371)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:820)
at com.zimbra.webClient.servlet.JspServlet.service(Js pServlet.java:50)
at org.mortbay.jetty.servlet.ServletHolder.handle(Ser vletHolder.java:487)
at org.mortbay.jetty.servlet.ServletHandler.handle(Se rvletHandler.java:362)
at org.mortbay.jetty.security.SecurityHandler.handle( SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(Se ssionHandler.java:181)
at org.mortbay.jetty.handler.ContextHandler.handle(Co ntextHandler.java:716)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebA ppContext.java:406)
at org.mortbay.jetty.servlet.Dispatcher.forward(Dispa tcher.java:268)
at org.mortbay.jetty.servlet.Dispatcher.forward(Dispa tcher.java:126)
at org.mortbay.jetty.servlet.DefaultServlet.doGet(Def aultServlet.java:465)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:707)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:820)
at org.mortbay.jetty.servlet.ServletHolder.handle(Ser vletHolder.java:487)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1093)
at com.zimbra.webClient.filters.SetHeaderFilter.doFil ter(SetHeaderFilter.java:264)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1084)
at org.mortbay.servlet.UserAgentFilter.doFilter(UserA gentFilter.java:81)
at org.mortbay.servlet.GzipFilter.doFilter(GzipFilter .java:148)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1084)
at org.mortbay.jetty.servlet.ServletHandler.handle(Se rvletHandler.java:360)
at org.mortbay.jetty.security.SecurityHandler.handle( SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(Se ssionHandler.java:181)
at org.mortbay.jetty.handler.ContextHandler.handle(Co ntextHandler.java:716)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebA ppContext.java:406)
at org.mortbay.jetty.handler.ContextHandlerCollection .handle(ContextHandlerCollection.java:211)
at org.mortbay.jetty.handler.HandlerCollection.handle (HandlerCollection.java:114)
at org.mortbay.jetty.handler.HandlerWrapper.handle(Ha ndlerWrapper.java:139)
at org.mortbay.jetty.handler.RewriteHandler.handle(Re writeHandler.java:176)
at org.mortbay.jetty.handler.HandlerWrapper.handle(Ha ndlerWrapper.java:139)
at org.mortbay.jetty.Server.handle(Server.java:313)
at org.mortbay.jetty.HttpConnection.handleRequest(Htt pConnection.java:506)
at org.mortbay.jetty.HttpConnection$RequestHandler.he aderComplete(HttpConnection.java:830)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser. java:514)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpPa rser.java:211)
at org.mortbay.jetty.HttpConnection.handle(HttpConnec tion.java:381)
at org.mortbay.io.nio.SelectChannelEndPoint.run(Selec tChannelEndPoint.java:396)
at org.mortbay.thread.BoundedThreadPool$PoolThread.ru n(BoundedThreadPool.java:442)
2008-08-25 14:48:50,972 ERROR [btpool0-10] [] log - /zimbra/
java.lang.NullPointerException
at com.zimbra.webClient.servlet.SkinResources.parseUs erAgent(SkinResources.java:658)
at com.zimbra.webClient.servlet.SkinResources.doGet(S kinResources.java:154)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:707)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:820)
at org.mortbay.jetty.servlet.ServletHolder.handle(Ser vletHolder.java:487)
at org.mortbay.jetty.servlet.ServletHandler.handle(Se rvletHandler.java:362)
at org.mortbay.jetty.security.SecurityHandler.handle( SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(Se ssionHandler.java:181)
at org.mortbay.jetty.handler.ContextHandler.handle(Co ntextHandler.java:716)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebA ppContext.java:406)
at org.mortbay.jetty.servlet.Dispatcher.include(Dispa tcher.java:192)
at org.apache.jasper.runtime.JspRuntimeLibrary.includ e(JspRuntimeLibrary.java:997)
at org.apache.jsp.public_.login_jsp._jspService(login _jsp.java:557)
at org.apache.jasper.runtime.HttpJspBase.service(Http JspBase.java:93)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:820)
at org.apache.jasper.servlet.JspServletWrapper.servic e(JspServletWrapper.java:373)
at org.apache.jasper.servlet.JspServlet.serviceJspFil e(JspServlet.java:477)
at org.apache.jasper.servlet.JspServlet.service(JspSe rvlet.java:371)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:820)
at com.zimbra.webClient.servlet.JspServlet.service(Js pServlet.java:50)
at org.mortbay.jetty.servlet.ServletHolder.handle(Ser vletHolder.java:487)
at org.mortbay.jetty.servlet.ServletHandler.handle(Se rvletHandler.java:362)
at org.mortbay.jetty.security.SecurityHandler.handle( SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(Se ssionHandler.java:181)
at org.mortbay.jetty.handler.ContextHandler.handle(Co ntextHandler.java:716)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebA ppContext.java:406)
at org.mortbay.jetty.servlet.Dispatcher.forward(Dispa tcher.java:268)
at org.mortbay.jetty.servlet.Dispatcher.forward(Dispa tcher.java:126)
at org.mortbay.jetty.servlet.DefaultServlet.doGet(Def aultServlet.java:465)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:707)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:820)
at org.mortbay.jetty.servlet.ServletHolder.handle(Ser vletHolder.java:487)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1093)
at com.zimbra.webClient.filters.SetHeaderFilter.doFil ter(SetHeaderFilter.java:264)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1084)
at org.mortbay.servlet.UserAgentFilter.doFilter(UserA gentFilter.java:81)
at org.mortbay.servlet.GzipFilter.doFilter(GzipFilter .java:148)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1084)
at org.mortbay.jetty.servlet.ServletHandler.handle(Se rvletHandler.java:360)
at org.mortbay.jetty.security.SecurityHandler.handle( SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(Se ssionHandler.java:181)
at org.mortbay.jetty.handler.ContextHandler.handle(Co ntextHandler.java:716)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebA ppContext.java:406)
at org.mortbay.jetty.handler.ContextHandlerCollection .handle(ContextHandlerCollection.java:211)
at org.mortbay.jetty.handler.HandlerCollection.handle (HandlerCollection.java:114)
at org.mortbay.jetty.handler.HandlerWrapper.handle(Ha ndlerWrapper.java:139)
at org.mortbay.jetty.handler.RewriteHandler.handle(Re writeHandler.java:176)
at org.mortbay.jetty.handler.HandlerWrapper.handle(Ha ndlerWrapper.java:139)
at org.mortbay.jetty.Server.handle(Server.java:313)
at org.mortbay.jetty.HttpConnection.handleRequest(Htt pConnection.java:506)
at org.mortbay.jetty.HttpConnection$RequestHandler.he aderComplete(HttpConnection.java:830)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser. java:514)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpPa rser.java:211)
at org.mortbay.jetty.HttpConnection.handle(HttpConnec tion.java:381)
at org.mortbay.io.nio.SelectChannelEndPoint.run(Selec tChannelEndPoint.java:396)
at org.mortbay.thread.BoundedThreadPool$PoolThread.ru n(BoundedThreadPool.java:442)