Product:
Cognos Controller 10.4
Microsoft Windows 2016 server
Oracle database 12c

Problem:
New upgraded database from Controller 10.3 to Controller 10.4, then the user can not login in to Cognos Controller 10.4. He get a error message.

Error message:
Error loading Main Menu
System.ArgumentException: Item has already been added. Key in dictionary: ‘6000’ Key being added: ‘6000’ at system.Collections.Hashtable.Insert(Objet key, Object nvalue, Boolean add) at Cognos.Controller.Forms.ControllerMDI.LoadMenu()

Solution:
Something when wrong in the DBCONV upgrade steps.
Take a new backup of the cognos controller 10.3 database and restore it on the new database server.
Go into Controller Configuration.
Mark the database connection you have restored.
Click on the test icon to ensure you have access.
Click on the Run icon, to start the Database Conversion Utility.
Click on connect.
Click on upgrade.
There should be no errors.
Click close.

Restart the Cognos Controller services to make it know about the database change.
Restart Internet Information server with the dos command: iisreset
Restart the Cognos Controller Components, by going to Control Panel\All Control Panel Items\Administrative Tools. Open Component Services. Expand Component Services – Computers – My Computer – COM+ Applications – IBM Cognos Controller Consolidation.
Right click and select “shut down”. Start it direct thereafter.

Ask the controller users to test to login again.