Xerox iMFP: Authentication failure with error "Unknown error unable to authenticate".

Question:

When attempting to authenticate at a Xerox iMFP I receive the below error on the screen.
“Unknown error unable to authenticate“

Answer:

Check that the Xerox iMFP Application Service is working as expected by using the below URL in a web browser.

 http://server_host_name/PharosXeroxiMFP/Authentication/ConvAuth.svc

If this fails then check and ensure that the .NET 4.5 "HTTP Activation" feature (listed on page 8 of the installation guide) is enabled on the server.