We have recently deployed our application with the addition of
AJAX.net for the last two weeks we have been suffering deadlock issues
with the application pool. We are in a farmed environment and our
application uses impersionation on a domain security account. We are
using a state server all applications are running the latest patched
version of .NET 2.0 with the latest service patch I have put in place
an app domain unhandled exception handler and found the following
stack associatied with our exception.

System.NullReferenceException: Object reference not set to an instance
of an object.
   at
System.Web.SessionState.SessionStateModule.PollLockedSessionCallback(Object
state)
   at System.Threading._TimerCallback.TimerCallback_Context(Object
state)
   at System.Threading.ExecutionContext.runTryCode(Object userData)
   at
System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode
code, CleanupCode backoutCode, Object userData)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext
executionContext, ContextCallback callback, Object state)
   at System.Threading.ExecutionContext.Run(ExecutionContext
executionContext, ContextCallback callback, Object state)
   at System.Threading._TimerCallback.PerformTimerCallback(Object
state)

I have only found reference to this issue in one other location
http://forums.asp.net/t/1132396.aspx I will attempt to un-install the
security patch but that seems like a very poor solution to this
problem.