Failed to register ServiceWorker

[rubenmoor]

 A bad HTTP response code (404) was received when fetching the script.

and

push.js:24 TypeError: Failed to register a ServiceWorker for scope ('https://mydomain.net/') with script ('https://mydomain.net/sw.js'): A bad HTTP response code (404) was received when fetching the script.

This is on Google Chrome. I receive a similar error on Firefox.

Indeed mydomain.net/sw.js yields 404, but I don't know what else to expect there ... how does this url show up in push.js when registering the service worker?