An unknown error occurred when fetching the script (Service Worker)

Before trying anything else, check whether your https certificate is invalid or not matching the url you are accessing.

E.g. In my case I was trying to access https://localhost with a certificate that was registered for another domain.

While hitting "proceed" would allow me to enter to the site, this exact error would be printed to the console:

An unknown error occurred when fetching the script

For me this error went away when i added sw.js to the cache upon install. Simply forgot to do that, but it solved the issue.

I had exactly the same problem, I spent an hour trying to figure it out and it turned out that I had another tab for the same origin left opened somewhere (so using the same shared Service Worker) that had the "Offline" checkbox left checked and that prevented another tabs from requesting sw.js for some reason.

It seems that the offline state is leaking from the tab within Service Worker scope while not being properly reflected nor managed by the other tabs than the one that was turned Offline first.

So make sure you have no other clients running the same Service Worker - you should be able to find them listed in DevTools > Application > Service Workers.