iTunes output goes exclusively to Airport Express when it should not My AirPlay setup is messed up since a few days : I have headsets plugged to an Airport Express (AE) and it used to play music synchronously with my macmini, as expected.  

One day, while I did not do anything special, the macmini stopped to play music as soon as I switched on the AE headset, as if the music stream was redirected exclusively to the AE.
Yet, iTunes is properly configured in multiple sources mode :

 

If I uncheck the Fabrice's Airport source from iTunes and leave Computer as the only source, the problem remains : music still played exclusively by the AE ! Only switching off the AE makes the macmini emits the music again.

I tried unplug/replug the AE with no success.
Next step would be to reinitialize it but I'm open to others suggestions.
By looking at the menu bar to see selected output device, I noticed that the Bluetooth icon was ON and thus interfering with Wifi Airplay.
By setting Bluetooth OFF on the mac mini, it corrected the problem.