Problem description Solution
HTTP 400 - Bad Request (Request header too long) Microsoft Knowledge Base 2020943 / Check, if there is a space in the name of the registry key
HTTP 503 Check status of Application Pool and start it, if stopped
Check, if password of Application Pool user is expired (Reset if necessary)
No connection could be made because the target machine actively refused it Run and exit Fiddler
User cannot consent to web app requesting user impersonation as an app permission Change delegated permission Access the directory as the signed-in user to Sign in and read user profile for Windows Azure Active Directory application