From what I understand, the fetch API may deliberately return very limited error details on failure. (Otherwise, a malicious JS program could issue a variety of fetch requests and look at their results to identify which internal servers exist, which servers the user has access to, etc.)

If you have access to a Mac, you can see this for yourself by experimenting with fetch requests from the Safari DevTools console. For example:

The request for google.com fails due to CORS. The request for example.does-not-exist is an invalid name. The DevTools console gives full details, so a developer who's actually at the browser can troubleshoot, but those details aren't available to JS code; the JS code gets the same "TypeError: Load failed" message in each case.

If you're lucky enough to have a way to access the browser console, you can see more details, but otherwise, I'm not aware of anything you can do to track down the problem - it could presumably be virtually any network error.