Error in agent's log indicates "Authentication failure; agent cannot authenticate endpoint server"
One possible cause for the problem is a network access control system, like NetMirror for example. NetMirror sends TCP reset packets to terminate any unauthenticated traffic.
Set the traffic between Endpoint Agent and Endpoint Server into a trusted list in the network access control system (NetMirror in this example). Otherwise, the network component of the network access control system will terminate the communication between them by sending TCP reset packets.
Note: Please contact Symantec Technical Support for further assistance if this is not the cause of your issue.