[ownedbycats]

For what it matters, HTTP status codes in the 5xx range are pretty much all serverside issues - - 500 Internal Service Error, 502 Bad Gateway, 503 Service Unavailable, 504 Gateway Timeout, etc etc. There isn't much to be done except wait for the site to be fixed.

The 4xx range are more likely client errors - so if you get 401 Unauthorized, 403 Forbidden, etc, that's when you'd want to check your FFF settings. (Though 404 Not Found and 410 Gone would indicate that the page isn't there.)