I've even tried a very minimal version of a sw and it gives me the same(-ish) errors:

Failed to load resource: net::ERR_BLOCKED_BY_CLIENT
SW registration failed with error AbortError: Failed to register a ServiceWorker: ServiceWorker cannot be started

I would like to attach it but I'm getting an error (rly? :P) when I click the Attach a file button below the comment box (see below) so I've uploaded it somewhere else:
https://stor4ge.rejh.nl/_stored/dump/temp/sw-simple-chromium-issue-848645.js

Uncaught TypeError: _fetchOptions is not a function
    at Array.<anonymous> (detail?id=848645&can=2&q=serviceworker fails to start&colspec=ID Pri M Stars ReleaseBlock Component Status Owner Summary OS Modified:1181)
    at window.onload (detail?id=848645&can=2&q=serviceworker fails to start&colspec=ID Pri M Stars ReleaseBlock Component Status Owner Summary OS Modified:74)
2detail?id=848645&can=2&q=serviceworker fails to start&colspec=ID Pri M Stars ReleaseBlock Component Status Owner Summary OS Modified:1020 Uncaught TypeError: _addAttachmentFields is not a function
    at HTMLAnchorElement.<anonymous> (detail?id=848645&can=2&q=serviceworker fails to start&colspec=ID Pri M Stars ReleaseBlock Component Status Owner Summary OS Modified:1020)

Just checked mobile.twitter.com to verify if I can reproduce this there. The answer is yes:

Failed to load resource: net::ERR_BLOCKED_BY_CLIENT
main.1dbc955161d4e61c.js:1 error-reporting: AbortError: Failed to register a ServiceWorker: ServiceWorker cannot be started {index: "responsive_web_prod", type: "ERROR", id: 1527859666223, source: {…}, tag: "DOMException"}

I got a different error in Chrome Beta, I guess it's related?

The FetchEvent for "https://DOMAIN/?gotoSection=onboard&gotoScreen=0" resulted in a network error response: the promise was rejected.

And then

Uncaught (in promise) TypeError: Failed to execute 'fetch' on 'ServiceWorkerGlobalScope': 'only-if-cached' can be set only with 'same-origin' mode

Which is weird because the request that failed (first line) *is* on the same origin?

I suspect this is not the cause, but can you try disabling Site Isolation just to confirm the problem still happens?  You can do this at chrome://flags#site-isolation-trial-opt-out, choosing "Opt-out (not recommended)," and restarting.

More info at:
https://www.chromium.org/Home/chromium-security/site-isolation#TOC-Diagnosing-Issues

(I'm guessing this is more related to actual ServiceWorker code than Site Isolation, but it doesn't hurt to rule it out.)

That doesn't work, unfortunately. On my first load I get:

Uncaught (in promise) TypeError: Failed to execute 'fetch' on 'ServiceWorkerGlobalScope': 'only-if-cached' can be set only with 'same-origin' mode

And when I reload:

Failed to load resource: net::ERR_BLOCKED_BY_CLIENT
Uncaught (in promise) DOMException: Failed to update a ServiceWorker: ServiceWorker cannot be started

The attachments in comment 0 are not complete so I can't test them (they don't include the importScripts contents).

The attachment in comment 2 works fine for me (if _log is defined to console.log) at https://crbug848645.glitch.me/. I suspect ERR_BLOCKED_BY_CLIENT has something to do with your environment. Based on other bugs, this error seems to have to do with extensions.

The error in comment 5 and 8 indicates an error in the service worker JavaScript code at a call to fetch(). This error message was added in Chrome 64. It says that Request.cache is 'only-if-cached' and Request.mode is 'same-origin', which is not a valid combination.  See:
https://developer.mozilla.org/en-US/docs/Web/API/Request/cache
https://developer.mozilla.org/en-US/docs/Web/API/Request/mode

To fix this error, change the cache property or the mode property to something else.

I'm not sure why this might have regressed, as there is no script to test on. Setting to WontFix but let me know if you can create a closed script that worked in an earlier Chrome and doesn't work with current Chrome.

> The attachment in comment 2 works fine for me (if _log is defined to console.log) at https://crbug848645.glitch.me/. I suspect ERR_BLOCKED_BY_CLIENT has something to do with your environment. Based on other bugs, this error seems to have to do with extensions.

Looks like it was one of the uBlock Origin filters that blocked the import of the firebase library. /facepalm

Right, I guess I'll try to contact the maintainer of that filter list and let him know about the issue. Thanks!