It used to work, and then some weeks ago, it suddenly stopped working.
I've tried reinstalling Spotify everywhere, rebooting the apple TV (which is on the latest firmware), the router.
From my laptop, I can ping the Apple TV IP, and traceroute says it's a single hop.
It works if I pair the Apple TV, and use it as a device "Not on the network", but then only I can use it and others can't.
It's the same issue from any (5 different) devices to the Apple TV, but it works fine between other devices on the same network, so it seems it's not the network.
With other Apple TV apps, local networking work correctly (e.g. with VLC)