SELECTserver tasks fail randomly without clear indication of why. The taks are usually the "send logs" or log posting and the "Data update".
Change the time the services are run. Another network service may be using the same resources that SELECTserver needs to send the logs The duration of this service is different from day to day so the problem occurs only when it is too long. Changing the time the services are run to different time when no other services run will solve the problem.