This is caused by a permission issue. Either the account which is being used to run the EventSentry console, or the account configured for Set Authentication on the affected host, is a valid account to log on but the account does not have Administrator permission for the affected host.
Another possibility is that the permission is correct but the authentication method is not accepted by the affected host. In the EventSentry console toolbar, click Tools > Options. On the Remote Update tab of the options, configure the Authentication Method Preference to be "Impersonation" rather than "IPC$". Save your changes and then attempt to check the agent status again, and the agent status should successfully display.