I have the same issue, can this be fixed please as one of the main reasons I use the product and can’t change or save the alarm through my mobile. 

I still have this problem ever since the major app update a few months ago. Extremely frustrating as I’ve spent hrs trying to resolve. A lengthy phonecall to Sonos help did not resolve. Please sort this out! 

Perhaps it’s something local blocking connection between the controller and Sonos product(s). I don’t use Sonos Alarms normally, as I tend to use Alexa Alarms built into Sonos instead, but just tried creating/saving two Alarms here quickly and both saved fine using the latest Sonos App.

See attached screen recording made a few moments ago. 

Damn that sucks. The app is still stupid buggy four months later so who knows what the problem could be.
 

But a possible fix. Double check that *.cdn.optimizely.com isn’t blocked on your device or network. Your controller needs to connect to this to enable missing features. If it is, delete the app and reinstall. You can block it again this after the reinstall. 

Thanks - I’m unsure how to check it’d that it is blocked, but I’ve deleted the app and reinstalled it several times from Apple App Store. The issue remains.

If perhaps not done already, it maybe best/simpler to reproduce the problem (with screenshots) and then immediately submit a Sonos system diagnostic report from within the App, note it’s reference and contact/chat with Sonos Support Staff via this LINK and see what the Staff can perhaps suggest to help you to resolve the matter.

I have the same issue with my Sonos system. Alarms will not save

Will it perhaps work if you power off your older products and just leave your newer Sonos One SL up and running - I’m just wondering if you’re perhaps running out of storage space on one of your older units - some memory might have been tagged as being defective by the OS if they units are getting old.