Product:
Cognos Controller 8.5.1
Microsoft Windows 2008 R2 server

Symptom:
When you start the Cognos Controller client from the link in Cognos Connection you get a error message.

Error message:
System.NullreferenceException: Object reference not set to an instance of an object.
at Cognos.Controller.AppContext.InitConfig()

Possible Solution:
Logon to the Cognos Controller server as administrator

Open folder C:\Windows\System32\inetsrv\config
Open file “ApplicationHost.config”
Change
to
Click Save

Restart the IIS server and test again