Internal Server Error in .NET 4, “Calling LoadLibraryEx on ISAPI filter”

I encountered an odd issue today.

I received an Internal Server Error for a .NET 4 application:

Calling LoadLibraryEx on ISAPI filter "C:\Windows\Microsoft.NET\Framework\v4.0.30319\\aspnet_filter.dll" failed

I looked around for a quick solution and found a few articles which suggested running aspnet_regiis -r from the Framework64 directory. This didn’t work.

Then, I noticed the path separator is doubled in the message. Removing this extra backslash fixes the issue.

To Fix in IIS 7:

  1. Hit WINDOWS KEY + R, and run inetmgr
  2. Highlight either your machine or site node in the left panel
  3. Choose ISAPI Filters from the middle panel
  4. Expand the ‘Executable’ column in the ISAPI Filters view
  5. Double-click the offending filter (in my case, ASP.NET_4.0.30319.0)
  6. Remove the duplicate ‘\’ and click ‘OK’
  7. Restart the service and everything should be good

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