won’t work until new app update, which will take weeks apparently.

welcome to the club!!

Yes azabeaux that was my guess too, but I keep trying and the next success was as I connected both speaker directly to the LAN and made the factory reset once more. I got a message my One SL needs an update and I could start and finish this update succesfully. Nobody knows why it does not work via WLAN. 

Now I see One SL in the system information :-)), so its just a question of hours to get it working like before ;-)
 

and you can use them again with wlan as surrounds?

how do you factory reset them?

Well, that was too good to be true. After I removed the LAN cable and switched to WLAN, the SL disappeared again.  
Again the whole thing from the beginning, LAN cable to both speakers and I see both speakers in the system information. But nothing can be started on the One SL, always a different error message, even an attempt to connect them as a stereo pair fails. And when I called support this morning, they said 50 minutes waiting time. Great!!!
I wish the old WORKING app would be available again :-((

I reset the Sonos One SL to the factory settings by switching off the power and holding down the "Connect" button (just above the power connection) while switching it on until the display flashes orange and white. As soon as the process is complete, the display flashes green and then connect the speaker again to the Sonos System.

I was trying to add new One SL into my system but I tried wlan, lan, factory reset, reset my app. Change from iPhone to iPad all doesn’t work. Sometimes I can add them but once it prompts me to

update error code -1,002. Omg it’s a nightmare

@maxelin i feel with you… sometimes its strange.

Today I try once again and now it works, God knows why. But I connect One SL via LAN and the other One stays via WLAN. If I try to cnnect both with WLAN - no way. I can’t understand it, but I am happy to hear my music again.

I tried lan as well. Does not work. The problem is not adding into the system. But when it prompts me to update then the error code appear

Hi guys - Sonos One SL’s - bit of a nightmare to be honest.  Using them as L and R Front Surrounds, with Arc, Sub and a pair of Sonos One’s as L and R Rear Surrounds.  All working fine until I noticed faint music from a playlist coming out of the SL’s, but other speakers were working great with TV on!  So, did factory re-set on the SL’s, but System App won’t allow me to ‘Add’, keeps going round and round saying ‘your Sonos One SL has been added but may not appear in the System Setting’  and so on, ad nauseum, as with other comments on here.  Tried Ethernet, WIFI, Re-boot, nada, zip, zilch.  Help offers appreciated, thanks!

I could connect One SL with factory reset, than LAN cabel and I was able to add this speaker to the system. But it don’t work without LAN (Ethernet cable) the speaker disappears if I try to change to WLAN. It ist my expirience and I hope it helps. But we know: the ways of the Lord are unsearchable, so don’t give up!