I have encountered the same problem using the 750-8000 as a Modbus master device (with any Modbus slave device). This continues to be an issue with FW04. Looking at Wireshark, the BC100 will send a single SYN telegram and will ‘hang’ in a busy state until this is acknowledged. This causes an issue when the Modbus slave goes offline as the telegram is never acknowledged.
Currently, the only work around I have found is to restart the BC100 or disable the Modbus auto-reconnect and ping the target slave device until it is back online. Once the ping request is returned consistently for a few seconds, then reset the Modbus communication.