Sporadic exceptions uploading an attachment

We are seeing this exception pretty often while uploading attachments (but not all the time). The file is not uploaded successfully after seeing this error, but trying again right afterward usually works fine. Still, we would still like to get to the bottom of this. Anyone have suggestions? - Peter Error number 0 in 11: Uncaught exception Wrapped Exception: null com.xpn.xwiki.XWikiException: Error number 0 in 11: Uncaught exception Wrapped Exception: null at com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:161) at org.apache.struts.action.RequestProcessor.processActionPerform(RequestPr ocessor.java:431) at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java: 236) at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196) at org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414) at javax.servlet.http.HttpServlet.service(HttpServlet.java:689) at javax.servlet.http.HttpServlet.service(HttpServlet.java:802) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(Applica tionFilterChain.java:237) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilt erChain.java:157) at com.xpn.xwiki.web.SetCharacterEncodingFilter.doFilter(SetCharacterEncodi ngFilter.java:121) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(Applica tionFilterChain.java:186) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilt erChain.java:157) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValv e.java:214) at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveCo ntext.java:104) at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:5 20) at org.apache.catalina.core.StandardContextValve.invokeInternal(StandardCon textValve.java:198) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValv e.java:152) at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveCo ntext.java:104) at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:5 20) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java :137) at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveCo ntext.java:104) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java :118) at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveCo ntext.java:102) at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:5 20) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve. java:109) at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveCo ntext.java:104) at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:5 20) at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:929) at org.apache.coyote.tomcat5.CoyoteAdapter.service(CoyoteAdapter.java:160) at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:79 9) at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processC onnection(Http11Protocol.java:705) at org.apache.tomcat.util.net.TcpWorkerThread.runIt(PoolTcpEndpoint.java:57 7) at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool .java:683) at java.lang.Thread.run(Thread.java:595) Wrapped Exception: java.lang.NullPointerException at com.xpn.xwiki.web.UploadAction.action(UploadAction.java:51) at com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:144) at org.apache.struts.action.RequestProcessor.processActionPerform(RequestPr ocessor.java:431) at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java: 236) at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196) at org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414) at javax.servlet.http.HttpServlet.service(HttpServlet.java:689) at javax.servlet.http.HttpServlet.service(HttpServlet.java:802) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(Applica tionFilterChain.java:237) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilt erChain.java:157) at com.xpn.xwiki.web.SetCharacterEncodingFilter.doFilter(SetCharacterEncodi ngFilter.java:121) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(Applica tionFilterChain.java:186) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilt erChain.java:157) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValv e.java:214) at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveCo ntext.java:104) at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:5 20) at org.apache.catalina.core.StandardContextValve.invokeInternal(StandardCon textValve.java:198) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValv e.java:152) at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveCo ntext.java:104) at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:5 20) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java :137) at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveCo ntext.java:104) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java :118) at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveCo ntext.java:102) at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:5 20) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve. java:109) at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveCo ntext.java:104) at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:5 20) at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:929) at org.apache.coyote.tomcat5.CoyoteAdapter.service(CoyoteAdapter.java:160) at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:79 9) at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processC onnection(Http11Protocol.java:705) at org.apache.tomcat.util.net.TcpWorkerThread.runIt(PoolTcpEndpoint.java:57 7) at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool .java:683) at java.lang.Thread.run(Thread.java:595)