New One SL's cannot be added to an S1 system.  See this https://support.sonos.com/en-us/article/sonos-s2-compatibility

Hi, after three hours of consulting the Sonos helpdesk, by phone, I have my system, with the Sonos One SL , working in Sonos S2 app.

I switched of my KPN Experia WIFI amplifiers, and used the WIFI switch on the Experia V9 box, to have only one WIFI signal in my house.

Now it works.

Later this afternoon, I will try to reconnect the two Experia WIFI amplifiers.

These amplifiers also make contact with my NAS, on which I have all the music I want to hear.

This NAS has to be connected to the network.

Best regards,

Bert

Hi.  The only change you have made that has made any difference is to move to S2.

Hi John,

Another important aspect is that I switched down the two KPN Wifi amplifiers.

That made the difference during installation.

Yesterday evening I connected the KPN Wifi amplifiers again and the Sonos systems (S1 and S2) still work.

(minus:  one Sonos Play 1 died during the upgrade proces)

Best regards,

Bertus.