Modbus TCP/IP Communication Stop

I work with Modbus TCP/IP Communication on CPU S7-1200.
I've configured MB_Server block V3.1, but after 15 min it stops. 
In MB_Server_DB , Static - Request_Count and Server_Message_Count stop to go up, and Connected stays TRUE.
After I STOP and then START the CPU, Request_Count and Server_Message_Count, and Connected are reset. And again it stops after 15 min.

Any ideas?

CPU - CPU 1214C DC/DC/DC.
Firmware - V4.0.

Thank you,
Olesia