It looks like the cipher suites supported by Minio are not compatible with Windows 8.1/Server 2012R2, if the private key is RSA. However, an ECDSA key is compatible, as then Minio can use the TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suite, which is also supported by these Windows versions.
Based on a discussion I’ve had with aead in the PR referenced above, the Minio documentation will soon be updated to describe how to generate ECDSA keys, but in brief, the command is openssl ecparam -genkey -name prime256v1 | openssl ec -out private.key.