Probleam reading from Arduino MKR Fox

Hello,

I loaded on my Arduino MKR Fox 1200 a sketch that sends information via the serial port but i don't get anything on my serial monitor.
Sometimes there's no error message at all, sometimes I get this:

processing.app.debug.RunnerException
	at cc.arduino.packages.uploaders.SerialUploader.uploadUsingPreferences(SerialUploader.java:160)
	at cc.arduino.UploaderUtils.upload(UploaderUtils.java:78)
	at processing.app.SketchController.upload(SketchController.java:713)
	at processing.app.SketchController.exportApplet(SketchController.java:686)
	at processing.app.Editor$DefaultExportHandler.run(Editor.java:2168)
	at java.lang.Thread.run(Thread.java:748)
Caused by: processing.app.SerialException: Erreur de la commande « touch » sur le port série « COM6 ».
	at processing.app.Serial.touchForCDCReset(Serial.java:107)
	at cc.arduino.packages.uploaders.SerialUploader.uploadUsingPreferences(SerialUploader.java:144)
	... 5 more
Caused by: jssc.SerialPortException: Port name - COM6; Method name - openPort(); Exception type - Port busy.
	at jssc.SerialPort.openPort(SerialPort.java:164)
	at processing.app.Serial.touchForCDCReset(Serial.java:101)
	... 6 more

I'm on Windows 7 64-bits.

I tried loading blink and it works, but whenever the sketch is supposed to communicate back, there's a problem.

Thanks in advance for your help!

Port name - COM6; Method name - openPort(); Exception type - Port busy.

means some other program on the computer has COM6 open.

How do i know what program could be using it?
It's weird that it can only communicate one way, like loading the code is possible but i cannot recieve anything back.