Sep 13, 2014 03:23 PM|hakenr|LINK
The symptoms you are facing usually occurs whenever there is an unhandled exception thrown in other than ASP.NET request thread. Do you have any fire&forget threads started by your application? Focus your diagnostics there...
(In worst case you will have to connect a debugger to your application and wait for the exception, i.e. using AD Plus or DebugDiag.)