WordPress runs an internal HTTP request to verify that a page‑cache layer is active. The request targets a core endpoint such as admin‑ajax.php and expects a successful 200 response with cache headers.
If the server blocks the request, returns 404/500, or times out, WordPress reports the loopback request problem. Common blockers include firewall rules, DNS resolution failures, disabled curl functions, or rewrite rules that redirect the health‑check URL.