cant remove Attributes from request scope

Hi, there:
I have a problem that I need your help. I spent lot of time trying to fix the problem, but no success.
The problem is:
When I run Struts in VAJ, I got the following error:---Message: Server caught unhandled exception from servlet [jsp]: cant remove Attributes from request scope.
The detailed error msg is:
Error 500
An error has occured while processing request:http://localhost:8080/struts/logon.jsp
Message: Server caught unhandled exception from servlet [jsp]: cant remove Attributes from request scope
Target Servlet: jsp
StackTrace:
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Root Error-1: cant remove Attributes from request scope
java.lang.IllegalArgumentException: cant remove Attributes from request scope
java.lang.Throwable(java.lang.String)
java.lang.Exception(java.lang.String)
java.lang.RuntimeException(java.lang.String)
java.lang.IllegalArgumentException(java.lang.String)
void org.apache.jasper.runtime.PageContextImpl.removeAttribute(java.lang.String, int)
int org.apache.struts.taglib.html.FormTag.doEndTag()
void _logon_xjsp._jspService(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
void org.apache.jasper.runtime.HttpJspBase.service(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
void javax.servlet.http.HttpServlet.service(javax.servlet.ServletRequest, javax.servlet.ServletResponse)
void org.apache.jasper.runtime.JspServlet$JspServletWrapper.service(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse, boolean)
void org.apache.jasper.runtime.JspServlet.serviceJspFile(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse, java.lang.String, java.lang.Throwable, boolean)
void org.apache.jasper.runtime.JspServlet.service(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
void javax.servlet.http.HttpServlet.service(javax.servlet.ServletRequest, javax.servlet.ServletResponse)
void com.ibm.servlet.engine.webapp.StrictServletInstance.doService(javax.servlet.ServletRequest, javax.servlet.ServletResponse)
void com.ibm.servlet.engine.webapp.StrictLifecycleServlet._service(javax.servlet.ServletRequest, javax.servlet.ServletResponse)
void com.ibm.servlet.engine.webapp.IdleServletState.service(com.ibm.servlet.engine.webapp.StrictLifecycleServlet, javax.servlet.ServletRequest, javax.servlet.ServletResponse)
void com.ibm.servlet.engine.webapp.StrictLifecycleServlet.service(javax.servlet.ServletRequest, javax.servlet.ServletResponse)
void com.ibm.servlet.engine.webapp.ServletInstance.service(javax.servlet.ServletRequest, javax.servlet.ServletResponse, com.ibm.servlet.engine.webapp.WebAppServletInvocationEvent)
void com.ibm.servlet.engine.webapp.ValidServletReferenceState.dispatch(com.ibm.servlet.engine.webapp.ServletInstanceReference, javax.servlet.ServletRequest, javax.servlet.ServletResponse, com.ibm.servlet.engine.webapp.WebAppServletInvocationEvent)
void com.ibm.servlet.engine.webapp.ServletInstanceReference.dispatch(javax.servlet.ServletRequest, javax.servlet.ServletResponse, com.ibm.servlet.engine.webapp.WebAppServletInvocationEvent)
void com.ibm.servlet.engine.webapp.WebAppRequestDispatcher.handleWebAppDispatch(com.ibm.servlet.engine.webapp.WebAppRequest, javax.servlet.http.HttpServletResponse, boolean)
void com.ibm.servlet.engine.webapp.WebAppRequestDispatcher.dispatch(javax.servlet.ServletRequest, javax.servlet.ServletResponse, boolean)
void com.ibm.servlet.engine.webapp.WebAppRequestDispatcher.forward(javax.servlet.ServletRequest, javax.servlet.ServletResponse)
void com.ibm.servlet.engine.srt.WebAppInvoker.handleInvocationHook(java.lang.Object)
void com.ibm.servlet.engine.invocation.CachedInvocation.handleInvocation(java.lang.Object)
void com.ibm.servlet.engine.srp.ServletRequestProcessor.dispatchByURI(java.lang.String, com.ibm.servlet.engine.srp.ISRPConnection)
void com.ibm.servlet.engine.oselistener.OSEListenerDispatcher.service(com.ibm.servlet.engine.oselistener.api.IOSEConnection)
void com.ibm.servlet.engine.http_transport.HttpTransportHandler.handleConnection(java.net.Socket)
void com.ibm.servlet.engine.http_transport.HttpTransportHandler.run()
void java.lang.Thread.run()

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Wrapped Error-2: cant remove Attributes from request scope
javax.servlet.ServletException: cant remove Attributes from request scope
java.lang.Throwable(java.lang.String)
java.lang.Exception(java.lang.String)
javax.servlet.ServletException(java.lang.Throwable)
void org.apache.jasper.runtime.PageContextImpl.handlePageException(java.lang.Exception)
void _logon_xjsp._jspService(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
void org.apache.jasper.runtime.HttpJspBase.service(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
void javax.servlet.http.HttpServlet.service(javax.servlet.ServletRequest, javax.servlet.ServletResponse)
void org.apache.jasper.runtime.JspServlet$JspServletWrapper.service(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse, boolean)
void org.apache.jasper.runtime.JspServlet.serviceJspFile(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse, java.lang.String, java.lang.Throwable, boolean)
void org.apache.jasper.runtime.JspServlet.service(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
void javax.servlet.http.HttpServlet.service(javax.servlet.ServletRequest, javax.servlet.ServletResponse)
void com.ibm.servlet.engine.webapp.StrictServletInstance.doService(javax.servlet.ServletRequest, javax.servlet.ServletResponse)
void com.ibm.servlet.engine.webapp.StrictLifecycleServlet._service(javax.servlet.ServletRequest, javax.servlet.ServletResponse)
void com.ibm.servlet.engine.webapp.IdleServletState.service(com.ibm.servlet.engine.webapp.StrictLifecycleServlet, javax.servlet.ServletRequest, javax.servlet.ServletResponse)
void com.ibm.servlet.engine.webapp.StrictLifecycleServlet.service(javax.servlet.ServletRequest, javax.servlet.ServletResponse)
void com.ibm.servlet.engine.webapp.ServletInstance.service(javax.servlet.ServletRequest, javax.servlet.ServletResponse, com.ibm.servlet.engine.webapp.WebAppServletInvocationEvent)
void com.ibm.servlet.engine.webapp.ValidServletReferenceState.dispatch(com.ibm.servlet.engine.webapp.ServletInstanceReference, javax.servlet.ServletRequest, javax.servlet.ServletResponse, com.ibm.servlet.engine.webapp.WebAppServletInvocationEvent)
void com.ibm.servlet.engine.webapp.ServletInstanceReference.dispatch(javax.servlet.ServletRequest, javax.servlet.ServletResponse, com.ibm.servlet.engine.webapp.WebAppServletInvocationEvent)
void com.ibm.servlet.engine.webapp.WebAppRequestDispatcher.handleWebAppDispatch(com.ibm.servlet.engine.webapp.WebAppRequest, javax.servlet.http.HttpServletResponse, boolean)
void com.ibm.servlet.engine.webapp.WebAppRequestDispatcher.dispatch(javax.servlet.ServletRequest, javax.servlet.ServletResponse, boolean)
void com.ibm.servlet.engine.webapp.WebAppRequestDispatcher.forward(javax.servlet.ServletRequest, javax.servlet.ServletResponse)
void com.ibm.servlet.engine.srt.WebAppInvoker.handleInvocationHook(java.lang.Object)
void com.ibm.servlet.engine.invocation.CachedInvocation.handleInvocation(java.lang.Object)
void com.ibm.servlet.engine.srp.ServletRequestProcessor.dispatchByURI(java.lang.String, com.ibm.servlet.engine.srp.ISRPConnection)
void com.ibm.servlet.engine.oselistener.OSEListenerDispatcher.service(com.ibm.servlet.engine.oselistener.api.IOSEConnection)
void com.ibm.servlet.engine.http_transport.HttpTransportHandler.handleConnection(java.net.Socket)
void com.ibm.servlet.engine.http_transport.HttpTransportHandler.run()
void java.lang.Thread.run()

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Wrapped Error-3: Server caught unhandled exception from servlet [jsp]: cant remove Attributes from request scope
com.ibm.servlet.engine.webapp.UncaughtServletException: Server caught unhandled exception from servlet [jsp]: cant remove Attributes from request scope
java.lang.Throwable(java.lang.String)
java.lang.Exception(java.lang.String)
javax.servlet.ServletException(java.lang.String, java.lang.Throwable)
com.ibm.websphere.servlet.error.ServletErrorReport(java.lang.String, java.lang.Throwable)
com.ibm.servlet.engine.webapp.WebAppErrorReport(java.lang.String, java.lang.Throwable)
com.ibm.servlet.engine.webapp.UncaughtServletException(java.lang.String, java.lang.Throwable)
void com.ibm.servlet.engine.webapp.WebAppRequestDispatcher.handleWebAppDispatch(com.ibm.servlet.engine.webapp.WebAppRequest, javax.servlet.http.HttpServletResponse, boolean)
void com.ibm.servlet.engine.webapp.WebAppRequestDispatcher.dispatch(javax.servlet.ServletRequest, javax.servlet.ServletResponse, boolean)
void com.ibm.servlet.engine.webapp.WebAppRequestDispatcher.forward(javax.servlet.ServletRequest, javax.servlet.ServletResponse)
void com.ibm.servlet.engine.srt.WebAppInvoker.handleInvocationHook(java.lang.Object)
void com.ibm.servlet.engine.invocation.CachedInvocation.handleInvocation(java.lang.Object)
void com.ibm.servlet.engine.srp.ServletRequestProcessor.dispatchByURI(java.lang.String, com.ibm.servlet.engine.srp.ISRPConnection)
void com.ibm.servlet.engine.oselistener.OSEListenerDispatcher.service(com.ibm.servlet.engine.oselistener.api.IOSEConnection)
void com.ibm.servlet.engine.http_transport.HttpTransportHandler.handleConnection(java.net.Socket)
void com.ibm.servlet.engine.http_transport.HttpTransportHandler.run()
void java.lang.Thread.run()
Please help me!!!
Nancy