Oh, Microsoft. By now, one might think that you’d learn not to push updates to systems without testing them thoroughly. One would be wrong. At least this one classifies as a minor annoyance and not outright breakage…
Windows Update offers up .NET 4.0 to Windows 2008 R2 systems as an Important update (and has been for a while). This is fine and good – various versions of the .NET framework can live in parallel. The problem, however, comes when you accept this update on an Exchange 2010 server with the CAS role.
If you do this, you may notice that the /exchange, /exchweb, and /public virtual directories (legacy directories tied to the /owa virtual directory) suddenly aren’t redirecting to /owa like they’re supposed to. Now, people aren’t normally using these directories in their OWA URLs anymore, but if someone does attempt to hit one of these virtual directories it leaves a gnarly error message to spam your event logs.
This is occurring because when .NET 4.0 is installed and the ASP.NET 4.0 components are tied into IIS, the Default Application Pool is reconfigured to use ASP.NET 4.0 instead of ASP.NET 2.0 (the version used by the .NET 3.5 runtime on Windows 2008 R2). What exactly it is about this that breaks these legacy virtual directories, I have no idea, but break them it does.
The fix for this is relatively simple: uninstall .NET 4.0 and hide the update from the machine so it doesn’t come back. If you don’t want to do that, follow this process outlined in TechNet to reset the Default Application Pool back to .NET 2.0. Be sure to run IISRESET afterwards.