Play:1 audio perfect from one source, skips/stops from other sources (wifi perfect)

It could be a fairly simple DNS issue with the SoundCloud/spotify servers. I’d try a network refresh, by unplugging your Sonos system from power, and rebooting your router. Wait two minutes, then plug back in your Sonos devices.

Alternately, you could submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

The unplugging of the Play:1 and router was done already many times :)

Also, I’m using the latest Sonos app on all phones.

Will try submitting the system diagnostic.

Meanwhile, if anyone has ideas what to debug further I’d be grateful!

If you didn’t have the Sonos powered off while you rebooted the router, you may not have received the full benefit. It’s unclear from your posts. But I’ll be interested to hear what support says. 

You are making a basic error that is likely to cause you much grief as you look for problems in a place that does not exist.

The Sonos controller is a remote control. Just like your TV remote control. You can power it down and there is no impact on the data as it does not pass through the remote control device.

Your issue is with the connection between your source and the Play 1.

Get the issue to happen, submit a diagnostic ASAP (as above) and call support to have it looked at.

Personally I'd suspect an issue with the cellular (SIM) router meeting Sonos' needs.

What do you mean exactly?

This is clear.

Yes, and the source is somewhere on the internet, and so we have internet → wifi router → Play:1. As I tried to describe the internet → wifi router connection is perfectly fine (we can stream netflix for example in HD) and the wifi router → Play:1 connection is also perfectly fine because using the Sonos app I can stream my own music library which is wired to the router. 

I submitted a diagnostic, chatted with a (supposedly) real person who just collected further info from me and told me to call them. Will do tomorrow.

What makes you think that? If we can stream netflix in HD on any device on the wifi network, why would the bandwidth not be enough for audio?

Sonos uses your network differently than most other devices, it is not uncommon to see posts here, like yours, saying the network is fine, but on closer examination finding there is a Sonos related issue not encountered by the other devices.

Raw speed, fast pings or other stuff working are not assurance that your Sonos will be happy on that connection.

I wish we users had more access to the internal Sonos data but since we don't support is your best hope.

Saying "it is your network"is both frustrating to the user and often wrong, better to say "there is a Sonos related issue with your network" and skip assigning blame while working to a solution.

 But still, your setup is listed here as unsupported and maybe Sonos has a reason for that.

https://support.sonos.com/en-us/article/sonos-system-requirements

Unsupported network setups and devices

Wireless internet connections such as satellite, mobile hotspots, or LTE routers