I fixed it!
I have got zero idea what it was that actually helped eventually, but now I see chromecast, airplay and spotify devices all the time in my network. Even when the receivd has been turned off for days. Once I start using any of them, the receiver automatically turns on and sets itself to correct mode.
At one point I tried to change what type of Spotify device it is. Not sure if that made any difference.
Next up, the recevier had huge issues once I installed my new Cisco Meraki WiFi access point. It was getting a new IP after every few minutes, though the DHCP server was the same as it had been before. I was so lost with the issue and the only thing left to try was to assign a static IP. Even that went very painfully. The WebUI sucks balls to be generous and eventually it was no help. I somehow managed to do it with OSD menus, but it was not something that was a success after a first try. No idea what is the secret recipe.
Either way, after I had done these everything is muy bien now and I almost feel bad that I have named the airplay device as "Stupid receiver".
