portserver TS4 disconnect port

Portserver TS4 works as TCP server. Serial device , connected to server port, sends messages with interval 30s. Keep-alived tcp packets portserver sends with interval 10s. But at one moment it stoppped. On other ports from TCP client’s side we send messages with interval 1s and connection exists always. What may be a matter?
We see sended and recieved packets with Wireshark.