leonardo compatible serial problems

Hi, I bought an Arduino Leonardo compatible Arduino Pro Micro with an ATmega32U4, but I have got problems using the serial communication.
I can upload my code without problems and the Arduino can also send me messages via the virtual com port. But if I want to send him data it doesn't receive any.
The LED does not light up and after the third sending attemp the whole arduino IDE hangs and stops working until I disconnect my board.

I've tried different computers (all windows 8.1), different usb-cables, as well as different serial consoles. Here is my code for the connection:

void setup() {
Serial.begin(9600);
while(!Serial){
}

}

void loop() {

}

Maybe someone had this problem before or has any advice for me.