HTTP status codes / 4xx — Client errors
This server is not the right one for that URL/authority.
The request reached a server that cannot produce an authoritative answer for it — typically an HTTP/2 connection-reuse artifact, where one TLS connection serves multiple hostnames and the request lands on the wrong backend.
Retry on a fresh connection; if persistent, the server’s TLS certificate/SNI setup spans hosts it should not.
Server-side note: Retry on a fresh connection; persistent 421s mean the cert/SAN list and routing disagree.
The fastest way to pin down a 421 is to reproduce the exact request and inspect what actually went over the wire — status, headers, timing and body, without your app code in the way. That is what an API client is for; ReqPad does it from your phone, with every request saved to history.
400 Bad Request · 401 Unauthorized · 402 Payment Required · 403 Forbidden · 404 Not Found · 405 Method Not Allowed — or the full reference.
Build the request, send it, read status + headers + timing — on your iPhone. Free to start.