Product:
Cognos Controller 10.3.1
Microsoft Windows 2012 R2 Server

Issue:
Error when you try to login to Cognos Controller client, after you have selected the database to use, and you have been logged in as a CAM user.

Error message:
System.Web.Services.Protocols.SoapException: Server was unable to process request. —> System.Exception: invalid_user

at Cognos.Controller.Proxy.SoapExtension.ServerExtension.ValidateCAMUser(SoapMessage& message)

at Cognos.Controller.Proxy.SoapExtension.ServerExtension.ProcessMessage(SoapMessage message)

at System.Web.Services.Protocols.SoapMessage.RunExtensions(SoapExtension[] extensions, Boolean throwOnException)

at System.Web.Services.Protocols.SoapServerProtocol.CreateServerInstance()

at System.Web.Services.Protocols.WebServiceHandler.Invoke()

at System.Web.Services.Protocols.WebServiceHandler.CoreProcessRequest()

— End of inner exception stack trace —

at Microsoft.VisualBasic.ErrObject.Raise(Int32 Number, Object Source, Object Description, Object HelpFile, Object HelpContext)

at Cognos.Controller.Common.RaiseErrCli.RaiseError(Int32 lErrNo, String sErrSource, String sErrDesc, String sErrHelpFile, Int32 lErrHelpContext)

at Cognos.Controller.Direct.ServerInfoD.GetServerInfo()

at CCR.AppContext.InitServerInfo()

at CCR.AppContext.Login()

Solution:
Login to your windows computer as a user who have rights to use Cognos Controller.
Then it will work. This user is not added inside Cognos controller, and therefor can not login.

Other solutions:
http://www-01.ibm.com/support/docview.wss?uid=swg21692450