If the planner screen has been idle for more than 20 minutes, the user will receive a session timeout error message.
To stop this error from occurring, go to the TM1 Web folder, open the web.config file and look for the following section:
The timeout parameter is in minutes and the default setting is 20. Alter this parameter to something more appropriate.
If your still having issues take a look at the latest TM1 (IBM Planning Analytics) release features.