Having the exact same problem.  I tried to get it to connect to my system and during the updating portion it just goes to the loading screen with the circle icon and it goes slower and slower and slower until it stops takes about an hour or so for that entire sequence.

Sonos support please help.  

I plan on contacting sonos now and trying to open a ticket.

Opened a ticket with Sonos.  Obviously its the new app problems.  But started a promising route and had to stop to make dinner last night.

1.  FACTORY RESET YOUR PLAYBAR.  (which I’m sure you’ve done a lot)
2. AFTER IT RESETS UNPLUG YOUR PLAYBAR
3. DOWNLOAD THE OLD SONOS APP S1
4. CLOSE ALL APPS
5. PLUG IN YOUR PLAYBAR
6. BOOT UP THE S1 APP
7. ADD THE PLAYBAR TO THE S1 APP BY MAKING A NEW SYSTEM
8. IT WILL PROPERLY UPDATE THE PLAYBAR
9. ---THIS IS WHERE I HAD TO MAKE DINNER---
10. BUT AT THIS POINT YOU COULD USE THE PLAYBAR TO WATCH MOVIES ETC.,
11. NEXT STEP IS TO UPGRADE FROM THE S1 APP TO THE NEW APP AND WHEN YOU DO THE UPGRADE FOR THE PLAYBAR IT SHOULD UPDATE THE SPEAKER.
12. The reason it is not updating from the new app is that the factory reset takes the playbar back to the setting from 2013.  That was 11 years ago.  It is too old and cannot update on the new sonos app so you have to first update it on the S1 app then transfer it over.  
13. Now I have not completed this conversion yet but plan on doing so tonight.
14. Let me know if you have success.

Having similar issue trying to add a Gen1 Sub to my existing system. New app wanting to update the sub but just goes slower and slower and I don't think it will ever finish. 

Any luck on your side ? 

Power down the Sub, hook it to Ethernet and your main router and try the update again.

Finally got it to accept the update, now here comes the next problem it won't connect to my play 5 (Gen2) stereo pair ?! it will only connect to my beam 

How did you complete the update in the end?

@Mface1 

I tried using my iPad Pro instead of my iPhone and it did the trick ‍♂️