SYMPTOM:
A user session can be manually disconnected by right-clicking on the user session and clicking on 'Disconnect' in Strategy Developer 9.x and 10.x, as shown below:

However, manually disconnecting user sessions of Source type 'Scheduler' fail with the following error message:

Error logging out user from project , it will be timed out after idling for 5 to 10 minutes.
CAUSE:
This is working as designed.
The user sessions will automatically time out as per the User Session Idle Time governor, set in the 'Configure Strategy Intelligence Server' dialog box. Therefore even if these sessions cannot be manually disconnected, these sessions will be removed by Strategy Intelligence Server once the User Session Idle Time Out value is reached.
ACTION:
An enhancement request has been logged to provide users the ability to manually disconnect 'Scheduler' sessions. Contact Strategy Technical Support for an update on the status of this issue.