I checked the logs in the browser.
After trying to open the Getting Started sample notebook, there were about 36 warnings that looked like more or less like The FetchEvent for the URL resulted in a network error response: the promise was rejected."
There were about 60 errors, and they had two main sorts: uncaught (in promise) TypeError: Failed to fetch at sw.js:[location], and failed to load resource: net::ERR_FAILED.
When I tried to open a new notebook, pretty much the same happened, I think: about 45 warnings of the same sort above, and 69 errors of the same sort mentioned above.