Hi! I also encountered the same problem and fixed it, finally. It displays "Connecting to /dev/ttyACM0 at 57600 baud" while the baud rate in my Arduino code was set to be 115200. Hence, set the baud rate as it was in your arduino code (In my case it is 115200) by adding _baud:=X (where 'X' is the baud rate between your computer and the Arduino) to the same command as follows:
rosrun rosserial_python serial_node.py _port:=/dev/ttyACM0 _baud:=115200