Session problem.

We have deployed sakai2.8.0.
we have enabled log, we have find this error many time, can any one tell us the problem.

2012-01-27 11:33:11,765 WARN ajp-8009-21 org.sakaiproject.portal.util.ErrorReporter - Bug Report bug-id: 25a1a3be-ed7d-47c7-9fcd-f6d77de30521 user: null usage-session: null time: Jan 27, 2012 11:33:11 user comment: null stack trace
org.sakaiproject.portal.api.PortalHandlerException: java.lang.IllegalArgumentException: The request must be not null
at org.sakaiproject.portal.charon.SkinnableCharonPortal.doGet(SkinnableCharonPortal.java:862)
caused by: java.lang.IllegalArgumentException: The request must be not null
at org.apache.commons.lang.Validate.notNull(Validate.java:192)
at net.sourceforge.wurfl.core.DefaultWURFLManager.getDeviceForRequest(DefaultWURFLManager.java:79)
at org.sakaiproject.portal.charon.SkinnableCharonPortal.startPageContext(SkinnableCharonPortal.java:1133)
at org.sakaiproject.portal.charon.SkinnableCharonPortal.sendPortalRedirect(SkinnableCharonPortal.java:1970)
at org.sakaiproject.portal.charon.SkinnableCharonPortal.redirectIfLoggedOut(SkinnableCharonPortal.java:1954)
at org.sakaiproject.portal.charon.handlers.ToolHandler.doTool(ToolHandler.java:117)
at org.sakaiproject.portal.charon.handlers.ToolHandler.doGet(ToolHandler.java:96)
at org.sakaiproject.portal.charon.SkinnableCharonPortal.doGet(SkinnableCharonPortal.java:862)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:627)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:269)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)
at org.sakaiproject.util.RequestFilter.doFilter(RequestFilter.java:659)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:215)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:172)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:174)
at org.apache.coyote.ajp.AjpAprProcessor.process(AjpAprProcessor.java:444)
at org.apache.coyote.ajp.AjpAprProtocol$AjpConnectionHandler.process(AjpAprProtocol.java:472)
at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1293)
at java.lang.Thread.run(Thread.java:619)

Tool Placement:
No Placement

Request:
: AuthType:null
: CharEncoding:UTF-8
: ContentLength:-1
: ContentType:null
: ContextPath:/portal
: LocalAddress:slateisb.nu.edu.pk
: LocalName:slateisb.nu.edu.pk
: LocalPort:80
: Method:GET
: PathInfo:/tool/CS206SPRING2012CS-home-02
: Protocol:HTTP/1.0
: QueryString:panel=Main
: RemoteAddress:111.68.106.76
: RemoteHost:111.68.106.76
: RemotePort:37145
: Requested URL:http://slateisb.nu.edu.pk/portal/tool/CS206SPRING2012CS-home-02
: Scheme:http
: ServerName:slateisb.nu.edu.pk
: Headers:
: Header:accept:image/gif, image/jpeg, image/pjpeg, image/pjpeg, application/x-shockwave-flash, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, application/xaml+xml, application/x-ms-xbap, application/x-ms-application, /
: Header:referer:http://slateisb.nu.edu.pk/portal/site/CS206SPRING2012CS
: Header:accept-language:en-us
: Header:user-agent:Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET CLR 2.0.50727; .NET CLR 1.1.4322; .NET4.0C; .NET4.0E)
: Header:accept-encoding:gzip, deflate
: Header:host:slateisb.nu.edu.pk
: Header:cookie:--censored--
: Header:Via:1.1 IT-Department:8080 (squid/2.6.STABLE21)
: Header:X-Forwarded-For:172.16.12.58
: Header:Cache-Control:max-age=259200
: Header:connection:keep-alive
: Parameters:
: Parameter:panel:0:Main
: Attributes:
: Attribute:sakai.character.encoding.done:sakai.character.encoding.done
: Attribute:sakai.filtered:sakai.filtered
: Attribute:sakai.session:org.sakaiproject.tool.impl.MySession@dc9fef7
Session:
: Created:1327645991765
: LastAccess:1327645991765
: MaxInactive:3600
: Attributes: