At our school there are (not smart) BenQ multimedia projectors in every classrom, every one of these is connected to an AppleTV (AppleTVs are not connected to WiFi) via HDMI cable and more and more colleagues are telling me that there seems to be no way to play a video stream (or a movie from the media library they own) containing image and sound via AirPlay or even via HDMI (using the official Apple adapter) now: the sound is perfectly fine, but there is only black rectangle instead of video even if the MacBook is connected directly to the beamer via HDMI.
So my question is: is there a way to stream a movie without using a Windows laptop?
YouTube (not YouTube Movies!) works perfectly fine btw.
Are our (quite) old projectors causing the problem?
Update: Looks like it's a DRM problem. VGA connection (via Apple adapter) works perfectly fine: the movie and its soundtrack are there.