warning occurring following move from classic to integrated on IIS7

I have migrated my c# asp.net web application from .net 3.5 to .net 4.0 and also changed the IIS7 application pool it runs to be Integrated rather than classic. The site runs fine but I have found the following warning been recorded in the server event log:

Event code: 3005 
Event message: An unhandled exception has occurred. 
Event time: 26/09/2011 11:19:10 
Event time (UTC): 26/09/2011 10:19:10 
Event ID: 5e750da6db8544feaede11ed88c072f6 
Event sequence: 2 
Event occurrence: 1 
Event detail code: 0 

Application information: 
    Application domain: /LM/W3SVC/3/ROOT-2-129615059474458846 
    Trust level: Full 
    Application Virtual Path: / 
    Application Path: << REMOVED >> 
    Machine name: << REMOVED >> 

Process information: 
    Process ID: 5864 
    Process name: w3wp.exe 
    Account name: << REMOVED >>\app_user

Exception information: 
    Exception type: NullReferenceException 
    Exception message: Object reference not set to an instance of an object.
   at System.Web.HttpApplication.set_AsyncResult(HttpAsyncResult value)
   at System.Web.HttpApplication.System.Web.IHttpAsyncHandler.BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData)
   at System.Web.HttpRuntime.P开发者_运维问答rocessRequestInternal(HttpWorkerRequest wr)



Request information: 
    Request URL: http://127.0.0.1/dummy.context 
    Request path: /dummy.context 
    User host address: 127.0.0.1 
    User:  
    Is authenticated: False 
    Authentication Type:  
    Thread account name: << REMOVED >>\app_user

Thread information: 
    Thread ID: 22 
    Thread account name: << REMOVED >>\app_user
    Is impersonating: False 
    Stack trace:    at System.Web.HttpApplication.set_AsyncResult(HttpAsyncResult value)
   at System.Web.HttpApplication.System.Web.IHttpAsyncHandler.BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData)
   at System.Web.HttpRuntime.ProcessRequestInternal(HttpWorkerRequest wr)


Custom event details: 

Any ideas what may be causing this as I am stumped as to what it might be?

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(tech difficulty: I'm having trouble to add a comment to yr question but it seems I can add it as an ansewer)

I am experiencing the same error. In this link https://jira.springsource.org/browse/SPRNET-1456 they say it's a bug that "won't be fixed"

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I had this error and looked in the Event Viewer. Among several other errors, I had this one and another (which was an Information) :

Information 14/11/2011 13:54:04 Windows Error Reporting 1001    None

details:

Fault bucket , type 0
Event Name: CLR20r3
Response: Not available
Cab Id: 0

Problem signature:
P1: w3wp.exe
P2: 7.5.7601.17514
P3: 4ce7afa2
P4: mscorlib
P5: 4.0.0.0
P6: 4e1823db
P7: f5
P8: 9
P9: System.StackOverflowException
P10: 

Attached files:

These files may be available here:
C:\ProgramData\Microsoft\Windows\WER\ReportQueue\AppCrash_w3wp.exe_426513163e392f7b5e2712abb67e65a456d0f5_015c843b

Analysis symbol: 
Rechecking for solution: 0
Report Id: b8a9aec0-0ebf-11e1-86d9-001d927c2d4c
Report Status: 0

So it was a stackoverflow. I guess that the error you see is just an artifact of another error. You should try to investigate for this other error. I don't say it will be a stackoverflow like mine, but it could be something else.