Please make sure that:
- Your TV is turned on before you start playback from Beamer.
- Your HDMI cable is fully connected. To be sure, please reconnect the cable. Even a slight disconnect can cause those HDCP errors.
In case that does not help, please try to restart your Apple TV.
It is also possible that your device does not support HDCP, but this is only true for very old TVs or projectors.