HTTP 421 Misdirected Request means the request was directed at a server that is not able to produce a response for the target URI. This can occur with HTTP/2 connection coalescing when a client reuses a connection for a domain whose certificate does not cover the requested hostname.
The client reused an HTTP/2 connection for a hostname not covered by the server's TLS certificate.
The server does not have a virtual host configured for the requested hostname.
Verify the server's TLS certificate covers the requested hostname (check Subject Alternative Names).
openssl s_client -connect example.com:443 -servername example.com 2>/dev/null | openssl x509 -noout -text | grep DNS:
Ensure the server has a virtual host configured for the requested domain.