Unable to connect after 'sonos-web start'

[degob]

Hello!
Thank you for the great idea of sonos-web!
Unfortunately I can't bring it to run.
I checked out the repo, ran npm install -g sonos-web-cli and tried to open http://localhost:5050. Unfortunately I just get unable to connect in my browser.
Do you have any idea, what can be the problem? Is there a log-file I could check? Am I missing something
My system: Ubuntu 20.04, Sonos S2 (App Version 12.0.2)
Thank you in advance!