FileMaker is really picky about the ECDSA SSL certificates it will recognize. So the EC key has to be based on the prime256v1 algorithm, and NOT the secp384r1 algorithm (for example). IOW:
openssl ecparam -genkey -name prime256v1 -out <keyfile.key> openssl req -x509 -sha256 -nodes -days 365 -in <keyfile> -out <certfile.crt>