Sometimes the system may display a message when trying to replay a session that 'session data is temporarily unavailable' and that one should try again later, the event is logged but data is not lost. This is mostly due to a cluster upgrade. Since sessions are stored on a single server, it can happen that if that particular node is restarting due to an upgrade that the session data is not available during that time. If no upgrade is running and this problem persists, try to check the load on the server
