nginx with 384-Bit Ecc Certificate and openssl but curve secp256r1 not usable

Solution 1:

Turns out P-256 has been removed from the list.

See:

Why Is TLS 1.3 an advancement over TLS 1.2 or 1.1?
TLS 1.3 removes support for known insecure ciphers such as RC4, DES, 3DES and export grade ciphers as well older hashing algorithms e.g. SHA-1 and MD5. These are welcome changes that should help to reduce the possibility of further vulnerabilities such as SWEET32 and FREAK being present within the code of TLS libraries e.g. OpenSSL.

This reduces the attack surface (defined within the second paragraph of this blog post) of TLS 1.3 but the improvements don’t stop there. Cipher suites such as NIST P-256 and AES-GCM are being removed as primitives with only x25519, ChaCha20 and Poly1305 remaining developed by Dan Bernstein (who uses the handle djb).

From here: securityinaction.wordpress.com