I did a little rewiring of all my cables connected to my TV, Apple TV , router etc.
While testing it I found out that my iTunes home sharing and Airplay was not working any more. My ATV did see my shared library but was not able to play the songs.
After consultation of a colleague and some troubleshooting I finally found the problem.
iTunes sharing uses the following ports:
A long time ago when I was doing some experiments with the PF firewall on OSX, I installed the Icefloor app.
Icefloor is a GUI frontend to the PF firewall. For more background on this app
I was thinking this Icefloor app was not running, but it was still active. It was blocking the essential ports 5353 and 3689.
I stopped the Icefloor app and home sharing is working again.