airplay from iMac to ATV 4 doesn't work

Hello,

I can not use Airplay on my iMac.

On my iMac there is an AirPlay icon available and do I have the Airplay option to choose in order to connect to my ATV 4.

But nothing happen.

When I go to the Airplay preferences I see "AirPlay Display:" is off but I can not turn it on.

With iPhone and iPad it works fine.

These hints on the following page were already checked.

Use AirPlay to display video from your Mac on an HDTV - Apple Support

El Capitan 10.11.5 (15F34)

Apple TV 4 iOS 9.2

Any idea to solve this issue?

Best regards