NullPointerException on container update

I get the following intermittant error:

Windows Server 2003
Jahia4.0.6_01
MySQL

On some occasions, a null pointer exception is generated when a user 
attempts to update content on a container.

ANy ideas as to what would cause this?

<!--
Error :

Error during handle of operation, Error during handle of operation, root 
cause:null


Exception StackTrace: Cause level : 0 (level 0 is the most precise 
exception)
java.lang.NullPointerException
	at 
org.jahia.engines.updatecontainer.UpdateContainer_Engine.initEngineMap(UpdateContainer_Engine.java:833)
	at 
org.jahia.engines.updatecontainer.UpdateContainer_Engine.handleActions(UpdateContainer_Engine.java:145)
	at 
org.jahia.operations.OperationManager.handleOperations(OperationManager.java:302)
	at org.jahia.bin.JahiaAction.execute(JahiaAction.java:50)
	at 
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:484)
	at 
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:274)
	at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1482)
	at org.jahia.bin.Jahia.process(Jahia.java:1545)
	at org.jahia.bin.Jahia.service(Jahia.java:860)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:809)
	at 
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:200)
	at 
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:146)
	at 
org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:209)
	at 
org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
	at 
org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
	at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:948)
	at 
org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:144)
	at 
org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
	at 
org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
	at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:948)
	at 
org.apache.catalina.core.StandardContext.invoke(StandardContext.java:2358)
	at 
org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:133)
	at 
org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
	at 
org.apache.catalina.valves.ErrorDispatcherValve.invoke(ErrorDispatcherValve.java:118)
	at 
org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:594)
	at 
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:116)
	at 
org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:594)
	at 
org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
	at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:948)
	at 
org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:127)
	at 
org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
	at 
org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
	at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:948)
	at org.apache.coyote.tomcat4.CoyoteAdapter.service(CoyoteAdapter.java:152)
	at org.apache.jk.server.JkCoyoteHandler.invoke(JkCoyoteHandler.java:300)
	at org.apache.jk.common.HandlerRequest.invoke(HandlerRequest.java:374)
	at org.apache.jk.common.ChannelSocket.invoke(ChannelSocket.java:743)
	at 
org.apache.jk.common.ChannelSocket.processConnection(ChannelSocket.java:675)
	at org.apache.jk.common.SocketConnection.runIt(ChannelSocket.java:866)
	at 
org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:683)
	at java.lang.Thread.run(Thread.java:534)