HTTP Error 500.21
Problem: User is getting the following error:
Error HTTP 500.21 – internal server error handler extensionlessurlhandler-isapi-4.0_64bit has a bad module isapimodule
Cause: The error occurs because the Aspnet_regiis.exe utility is no longer used for installing and uninstalling ASP.NET 4.5 on Windows 8 and Windows Server 2012 and above. ASP.NET 4.5 is now a Windows component and can be installed and installed just like any other Windows component.
Solution 1: If you are not using Windows 8.0 or Windows Server 2012 and above, or your server is using IIS 7 or below, then try the following command from the elevated command prompt:
Solution 2: If you are using Windows 8.0 or Windows Server 2012 and above, or your server is using IIS 8, then you will need to log in using a Windows Administrator account to uninstall all ASP.NET 4.5 IIS role services, reboot your server and then reinstall them.
Note: Please use the link given below as a reference to how to uninstall/reinstall APS.NET 4.5 IIS role services:
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