Heartbeat has been stopped
The Zentitle lan daemon hands out "seats" to client machines. When a client receives a seat, it initiates a heartbeat with the daemon. This error message indicates that the client machine dropped out of contact with the daemon without terminating the heartbeat; the error occurs after a number of missed heartbeats.
Solution
As the clients use network seats, they act much like a concurrent license on the Zentitle server. NSLReturnLicense() should be called on the client before closing the library with NalpLibClose(). The NSLReturnLicense() call will properly shutdown the heartbeat.