Hi @roelfsema
I use alarms on my Sonos One without any problems and I don’t use the Sonos App. The Sonos One allows either Alexa or Google voice assistant. I use Alexa and set all alarms (and snooze) via voice.
Commands
- Alexa set Alarm for every Week Day @ 5AM
- Alexa set Alarm for Weekends @ 6AM
When alarm goes off:
- Alexa Stop or Alexa Snooze (then Alexa Snooze or Stop)
You can submit a diagnostic, post the reference ID in this forum and Call Sonos or contact them via Twitter or Facebook. If you feel adventurous you cancel all Alarms in the Sonos App and try the following 
- Unplug you Sonos One
- Reboot your router
- Power Cycle your device
- Make sure you Sonos App and controller are up -to-date
- Plug in your Sonos One
- Set alarms via your chosen voice assistant.
Links:
Set up Amazon Alexa with a voice-enabled Sonos speaker
https://support.sonos.com/s/article/3517?language=en_US
Set up the Google Assistant with a voice-enabled Sonos product
https://support.sonos.com/s/article/3467?language=en_US
Let us know how things sort out. Cheers!