@zhenech Seems pretty clear to me: You can't KEX with x25519. You can KEX with ECDH over secp256r1. The host key is ECDSA using secp256r1.
x25519 kex isn't allowed in FIPS. ECDH over secp256r1 is. Host key signatures using secp256r1 are also OK, signatures using ed25519 only recently and your implementation may not have caught up with that yet.