Cannot connect to mBot after using Arduino IDE
I received some really good piece of advice on the official forum and it solved my problem, it now works:


  When you go back to using the mBlock (Scratch) environment you will
  need to reload the firmware (Upgrade firmware from the Connect menu
  item). At least that’s what I’ve had to do. Oh, and make sure that you
  have shut down the Arduino environment to release the serial port.