Product:
Cognos Controller 10.2.1
Windows 2012 R2 Server

Problem:
When a user that is logged into Cognos Controller clicks on the Excel icon to start excel from Cognos Controller, there is a message that the ADM user is already logged in.

Solution:
Somehow the user is noted as still logged in into Excel even do the program is closed.
Ensure all users are logged out of Cognos Controller by checking Maintain – Users – Active Users.
Go to the Controller server and restart the service “IBM Cognos Controller User Manager”.
This will throw out all users, even stuck ones, and you should be able to login again.