I’m having the same issue as well. 

Fresh Ace out of the box and same issue.

Yes, I'm in the same boat this morning.  Fresh headphones out of the box won't update; getting the same error repeatedly you mentioned.  Current headphone firmware is 2.6xxx.  Tried to update the headphones via my iPad and then Samsung Android phone (S22 Ultra), but no luck.  Very disappointing.  Thought Sonos would at least make sure the new app worked for the Ace, especially after all the updates since the app's early May release.  Isn't the Ace why they did the major software/app overhaul in the first place?!  All my previous Sonos products 'just worked' out of the box, updated fine, et al.  I'm happy to return these $450 headphones to Best Buy if this situation continues.  Absolute fail from Sonos.

same :( 

Mine arrived yesterday and are working well.

Upon initial setup this Ace software updated to 2.9.2.xxx and today updated to 2.9.3.xxx

Update takes seconds to download and 10+ minutes to install and restart on Ace 

Maybe try putting your update iPhone on Wi-Fi 2.4 mhz, that's my good luck charm.

Sadly I am having the same issues.  Headphones arrived today and unable to update :-( 

Had the exact same issue but found the solution.
 

Delete your iPhone Sonos app and reinstall it.

The update worked perfectly after. 

Had the same problem. Deleted the app and reinstalled. Update worked. iOS

Deleting the iOS app and reinstalling it fixed the problem.  Headphones now have the v.2.9.xxxx firmware and I was able to link it to my Sonos Arc.  Will now play with the headphones this evening. Thanks for posting the solution; Sonos itself wasn’t being very helpful.

Hello hello. I got my Ace headphones yesterday. Update required but I could not updated using my Android device. I tried my iOS device and it was successful. Kinda messed up that it works in one platform and not the other. 

I returned mine. Same issue with update . Also can't set up in app across different devices ,it only shows in the device I set it up on. Crackle in the right ear,and 10 days with no reply from Sonos support. 

I can't get mine to update to the latest software for the Ace either. It goes into a non-stop loop in the app when I attempt.

Tired uninstall and reininstall of the app, but it made no difference. That's on an Android phone.