This suggests that the TCP connection to the Phoenix app was established, but it was closed before any TLS handshake messages were sent by the server. This usually means there is a problem with your certificate/key files: Erlang’s :ssl does not verify the locations at startup, but only once the files are actually needed during the handshake. If it can’t read the files, the ssl socket crashes, and curl reports what you see.
So check if the env vars are correct, the files are present and readable and the private key is not encrypted (password protected).
