Potentially, your hosting infrastructure needs an upstream origin, despite you intending never to use it:

That's why we've set up https://error.fab.dev to use in these situations:

A properly-deployed FAB guarantees to respond to all requests, so the above page should never be reached. If it is, you know that the problem lies in routing the requests to the FAB in front of the origin.

For more discussion around this issue, see https://github.com/fab-spec/fab/issues/103