This applies to Windows Server 2003. This error can occur when you have .NET 1.0 and .NET 2.0 applications running on the same server.

This particular error can occur when you haven’t selected .NET 2.0. in the application settings.

Windows 2003 Uses Application Pools which can only support one version of the .NET framework at a time. If you are running both 2.0 and 1.0 applications on the same 2003 server then you have to create at least two application pools - one for each version of the framework. All of your 1.0 apps should be configured to use one pool and the 2.0 apps will use the other. The pool itself isn’t configured to specify which version it will support, but the app that starts first will “grab” the pool and lock it down to whatever version of the framework that particular app uses.  So if you have a 1.0 and 2.0 in the same pool, it will be a race between the two apps to see which can grab the pool first.  The winner will run fine and the loser will crash.  When IIS restarts, the race starts again.

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