When the T100MD888+ communicate with the E10 the PLC is the master of the network, and when you connect the PC to the same network then you are interfering with the communication between the two PLCs. TRiLOGI also pick up messages sent between the two PLCs which is not an intended response from a slave, so TRiLOGI has to close the connection and throw an exception. This is perfectly correct behavior.
You can only monitor the T100MD888+ PLC via its RS232 port since its RS232 port is now busy working with the E10.
Alternatively, you can connect a switch to a T100MD888+ digital input, and when you turn ON the switch the T100MD888+ program will stop sending data to the E10-npn+. Then the TRiLOGI software will be able to monitor the PLC properly without facing interference.