There is some records that the LOGOUT_TIME column is NULL in the session_log table. What situation does the LOGOUT_TIME column is null without being set a specific time to the LOGOUT_TIME column?
The LOGOUT_TIME column should be set the logout time after a user logoff the web interface. However, when a user closes the web interface by clicking X button, the LOGOUT_TIME column will be NULL until the timeout(TIMEOUT parameter in the web.cfg). When the timeout is come, the LOGOUT_TIME column will be set the time.
But if the SDM service is stopped(recycled), since the SDM does not address the operation for the timeout by internal, the LOGOUT_TIME will keep being NULL.
This is a known behavior, not a bug.