Playing from local music library gives exactly the same problem → Spotify is innocent. It seems like the Sonos App (running on a Apple Mini/OSX) looses connection to the Sonos Amp after 30-90 seconds. I have no clue why. Anyone?

Sonos app loosing connection does not make your music stop or skip; the app is merely a remote control.

That the problem exists for both streaming and playing local files indicates that the problem is within your network. Have you checked if you have wifi interference? https://support.sonos.com/s/article/3286?language=en_US

Did you follow Sonos’ guide for connecting a new router? https://support.sonos.com/s/article/1061?language=en_US

In addition to Sonos’ guide I recommend to restart the router, while your Sonos devices are powered off.

If none of above helps, try another ethernet cable, or try connecting only one Sonos by ethermet. Sonos will then create a wifi of its own, SonosNet, for Sonos alone.

Thank you, ClausN. I have now tried all your suggestions and made sure to follow the Sonos-guides you linked. I have also tested setting up a Wifi-network on the router for Sonos only, but the problem remains. The setup I continue testing on is only one unit connected (Sonos Amp) with a cabled connection.

What I observe is that the Sonos-units loose connection to the router after 110-120 seconds. As they re-connect, the Sonos unit jumps to the next song in current playlist. A struggle to see next step in my fault finding 

Shared diagnostics again with Sonos: ID 91858382

So I have no further suggestions, except calling Sonos Support when they open - unless a Sonos employee replies to this thread or another forum member replies with a solution.

https://support.sonos.com/s/?language=no

OK. Thanks for your efforts, ClausN!

Håkon