The root element is missing

Problem:

User launches the Controller client. User receives an error.

After pressing "Details" the first line of the error messages reads:
"System.Xml.XmlException: Root element is missing."

Resolving the problem:

Method #1 (local Client):

Browse to the end user's %APPDATA% location

TIP: By default, this is located here: C:\Users\<USERNAME>\AppData\Roaming\

Delete the "Cognos" Subfolder

Method #2 (Citrix / RDS Client):

Reset user profile