Why do I get the IIS error 500.21 when I start the web-based administration frontend?

The IIS error "HTTP Error 500.21 - Internal Server Error - Handler "PageHandlerFactory-Integrated" has a bad module "ManagedPipelineHandler" in its module list." appears if you try to open the administration frontend. This happens if the installation of ASP.Net encountered an error. You can fix this error the following way:

Execute the following command in the Command Prompt (run as administrator)
For 64 Bit operating systems: %windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_regiis.exe -i
Für 32 Bit operating systems: %windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_regiis.exe -i

Restart your IIS service and the XPhone Server service after you are done.