The Fastly CDN will sometimes return standardized 400 responses due to various issues that can occur when attempting to fetch data from your origin servers. The generic status text associated with a 400 error is "Bad Request" and typically indicates situations when an error seems to have been caused by the client when working with Fastly services. This page details the most common non-generic, standardized 400 responses and basic explanations of what each means at Fastly.
Error 421 Misdirected Request
The 421 Misdirected Request status code (RFC 9110) indicates that the request was directed at a server that is not able to produce a response. At Fastly, it typically indicates a mismatch between the hostname provided in the TLS handshake (e.g., in the SAN entries in the TLS certificate) and the requested hostname in the HTTP header. If you receive this error, contact Fastly support for assistance.