Cannot create new connection: client key rejected as invalid PEM PKCS#8 format

[GabrieleDeGuglielmo]

Hi,
I'm having trouble creating a new MQTT connection in Ditto. The error message returned is:

{
  "status": 400,
  "error": "connectivity:connection.configuration.invalid",
  "message": "/credentials/key: bad format. Expect PEM-encoded PKCS #8 data specified by RFC-7468 starting with '-----BEGIN PRIVATE KEY-----'",
  "description": "Please format your client key as PEM-encoded unencrypted PKCS #8."
}

Here’s what I did:

I generated the private key using the following OpenSSL command:
openssl ecparam -genkey -name prime256v1 -out device-key.key

I then converted the key (e.g., using KeyStore Explorer) and confirmed that:

• The algorithm is EC (P-256)

• The key size is 256 bits

• The format is PKCS#8

In the connection JSON, I’m passing the key as:
"key": "-----BEGIN PRIVATE KEY-----\nMIGHAgEAMBMGByqGSM49...3YEiS\n-----END PRIVATE KEY-----"

Despite this, Ditto still rejects the key format.

What am I doing wrong? Is there a specific way I need to generate or convert the EC private key so that Ditto accepts it as valid PEM-encoded unencrypted PKCS#8?

Thanks in advance for any help!