Troubleshooting IIS AppPool crashes (status 503) after Windows 10 Anniversary Update

IIS, Troubleshooting, Windows 10, Update

Installing the recently released Anniversary Update (version 1607) of Windows 10 seems to destabilize IIS by shutting down Application Pools, thus resulting in a 503 error when trying to run an application, which is caused by some DLLs failing to load when the worker process starts. Fortunately, the lovely folks of the interwebz are coming to the rescue!

First, check out the Windows Event Viewer (Win+X, V) to see what you need to fix: Open "Windows Logs", then "Application" and look for "Error" level entries with the source "IIS-W3SVC-WP" (may be different if the name of your IIS instance is not the default one). In the details, you will see a short message, like this:

The Module DLL <path-to-DLL> failed to load.  The data is the error.

Depending on your configuration, there may be different DLLs causing this kind of problem and they will occur one by one, so you will need to keep checking the Event Logs and fix the issues until your application properly starts up. To be sure, before every attempt, stop IIS and close IIS Manager.

Here are two specific issues we've experienced so far and how to fix them, but you may bump into completely different ones:

  1. "C:\WINDOWS\system32\inetsrv\rewrite.dll" (reference)
    • Go to "Programs and Features" (Win+X, F) and repair "IIS URL Rewrite Module 2".
  2. "C:\WINDOWS\system32\inetsrv\aspnetcore.dll" (reference)
    • Go to "Programs and Features" (Win+X, F) and repair "Microsoft .NET Core 1.0.0 - VS 2015 Tooling ...".

Happy updating!

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