[?]: s7-1200 Communication with cmus error ?

[NBachvarov]

Hello I
I have an error for s7-1200 plc that control chiller system .the error is Communication with cmus error ?
Can someone explain me what kind of error is this.
Is it internal error in the program.
I have this error when I try to put operation of chiller from manual to auto mode so the all pump and compressor to start automatically.
Just for information the chiller plc s7 1200 is connected whit another system ( Inert gas generator ) that is controlled with mitsubishi plc.