Many thanks for your suggestions!!! I'll start measuring, but it will take some time.
Just one thing - When I used the same hardware = USB Dongle, I got the correct response from the pioneer when I used Putty (sending "44VL"(both with and without XON/XOFF) -> resonse "VOL44"; But when I used the serial monitor in Arduion IDE (Using the same dongle - i.e. no change in hardware, my max232 and the Arduino were thus not connected) - I got a response in the Arduino IDE serial monitor "E04" -> which is the correct response when the sent command is wrong -> There was thus a connection between the Arduino IDE serial monitor and the pioneer (as "E04" was recived from the pioneer) -> but the command sent from the Arduion IDE must has been wrong.
Would the fact that the comunication, with identical hardware worked from the Putty, but not from the serial monitor in Arduino IDE indicate that there is something strange with the comunication protocol?