AccessViolationException, IIS

we are 4 developers (not professional server administrators) and running 2 servers with IIS 8.5 (Windows server 2012 R2) and ASP.NET 4.

Multiple times per day we get this exception in the windows event log:

Application: w3wp.exe
Framework Version: v4.0.30319
Description: The process was terminated due to an unhandled exception.
Exception Info: System.AccessViolationException
Stack:
   at System.Web.Hosting.UnsafeIISMethods.MgdIsLastNotification(IntPtr, System.Web.RequestNotificationStatus)
   at System.Web.Hosting.UnsafeIISMethods.MgdIsLastNotification(IntPtr, System.Web.RequestNotificationStatus)
   at System.Web.HttpRuntime.FinishRequestNotification(System.Web.Hosting.IIS7WorkerRequest, System.Web.HttpContext, System.Web.RequestNotificationStatus ByRef)
   at System.Web.HttpRuntime.OnRequestNotificationCompletionHelper(System.IAsyncResult)
   at System.Web.HttpRuntime.OnRequestNotificationCompletion(System.IAsyncResult)
   at System.Web.HttpApplication+PipelineStepManager.ResumeSteps(System.Exception)
   at System.Web.HttpApplication+CallHandlerExecutionStep.OnAsyncHandlerCompletion(System.IAsyncResult)
   at System.Threading.Tasks.Task.Execute()
   at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(System.Threading.Tasks.Task ByRef)
   at System.Threading.Tasks.Task.ExecuteEntry(Boolean)
   at System.Threading.ThreadPoolWorkQueue.Dispatch()

The next entry in the event log is this:

Faulting application name: w3wp.exe, version: 8.5.9600.16384, time stamp: 0x5215df96
Faulting module name: iiscore.dll, version: 8.5.9600.17224, time stamp: 0x53b1e5a1
Exception code: 0xc0000005
Fault offset: 0x0000000000002c44
Faulting process id: 0x257c
Faulting application start time: 0x01d05f0c93813e6e
Faulting application path: c:\windows\system32\inetsrv\w3wp.exe
Faulting module path: C:\Windows\system32\inetsrv\iiscore.dll
Report Id: 073f9a19-cb13-11e4-80c3-0cc47a02ce53
Faulting package full name: 
Faulting package-relative application ID: 

The always occur as a pair. I spent hours with google but have no idea if this is a problem in our code or in an external module and how to solve is. The main problem is that one of our servers stops responding from time to time, so I am really interested if this problems belong together.

We log all exceptions, but I cannot find it in our log. I also tried to attach the remote debugger, but I dont get the exception and the debugging really slows down the website.

Edit: I checked all sources to catch exceptions in m application:

• AppDomain.CurrentDomain.UnhandledException
• TaskScheduler.UnobservedTaskException
• Custom Owin Middleware

But I could not find this exception. The next thing I am gonna try is a custom http module. Lets see.

I also found some not warnings in the event log like:

Exception type: HttpException 
Exception message: The remote host closed the connection. The error code is 0x80070057.

or

Exception type: HttpException 
Exception message: A potentially dangerous Request.Path value was detected from the client (:).

These can happen very often, I am confused why they are added to the event log.