I\'ve uploaded a sketch to an Arduino Uno whose loop is something like this:
void loop(){
Serial.println(\"Hello, World!\");
}
So, now,
Based on my experience with the communication already in use or blocked, I would say that the program you are interfacing with still has the communication open.
I also found that if you disconnect the USB cable it will rest the communication. It is not the greatest solution, but it solves the problem.