Error "The request failed with HTTP status 401: Unauthorized." (52856)

Error "The request failed with HTTP status 401: Unauthorized." when accessing the Quest Knowledge Portal or Quest Knowledge Portal.

This is caused when the SQL Reporting Services (SRS) and Quest Knowledge Portal are installed on separate servers and the  Quest Knowledge Portal is accessed remotely. The user is unable to authenticate due to an issue with 2-hop (aka double-hop) authentication.

1. Trust the Quest Knowledge Portal server for delegation in Active Directory.
2. Ensure the Quest Knowledge Portal server and SQL Reporting Services server are in the internet browsers local intranet zone.
3. Check if SQL Reporting Services is running under Local System or if configured to run as a user account. If Local System, NTLM and Negotiate authentication methods should have been set by the SQL installer but check RSReportServer.config to make certain.
4. If SQL Reporting Services is running under a user account then it will have only been configured to accept NTLM authentication which is a problem if double hop authentication is needed. Negotiate will need to be added to the RSReportServer.config file like so:

AuthenticationTypes>
   <RSWindowsNegotiate/>
   <RSWindowsNTLM/>
  </AuthenticationTypes>

In some cases,   "<RSWindowsNTLM/>" may need to be remove from the rsreportserver.config file so that it appears as:

5. Only if SQL Reporting Services is running under a user context, create HTTP Service Principal Name(s) on the domain service account using Setspn.exe for the computer running SQL Reporting Services. Register both short name and FQDN to ensure both allow authentication.

Example:
CMD: Setspn -A HTTP/<System Name> <Domain\Service Account>