Product:
Cognos TM1 10.2.2
Windows 2008 R2 Server
Cognos Controller 10.1.1 FAP service

Problem:
Suddenly the FAP client can not login to the TM1 FAP instance, that have worked before.

Error message:
Could not login to TM1, host: servername.domain.com, server name: FAP user name: AD\Roger
SystemServerClientNotFound

Solution:
If you restart the TM1 instance, then you need to restart the FAP service.
1. Stop the FAP Service
2. Stop the TM1Service
3. Start the TM1Service
4. Start the FAP Service

Make sure to pause between the startup of TM1 and FAP, to ensure that the TM1 is up complete and ready to accept calls from clients.

More Information:
http://www-01.ibm.com/support/docview.wss?uid=swg21599573