Ok, I have figured out what was wrong: I was trying to keep compatibility with the "old" JSON "wallet" format.

[Michaelalmada]

          Ok, I have figured out what was wrong: I was trying to keep compatibility with the "old" JSON "wallet" format. 

Passing the credentials as a Buffer instead of a string into the identity property to the gateway's connect function solves the problem.

Here is the code snippet for those who want to keep the compatibility back with the "User.id" JSON wallet file format (assuming 2 env vars: one to configure the wallet path and another the identity name to be used to transact, plus using more or less the same code structure used into asset-transfer-basic/application-gateway-typescript sample):

const walletPath = process.env.WALLET_PATH || '/opt/node/wallet';
const identity = process.env.IDENTITY || 'Admin';

const walletFile = fs.readFileSync(`${walletPath}/${identity}.id`, 'utf8');
const wallet = JSON.parse(walletFile);

const newIdentity: any = async () => {
    const mspId = wallet.mspId;
    const credentials = Buffer.from(wallet.credentials.certificate); // here is the "gotcha"
    
    return { mspId, credentials };
}

const newSigner: any = async () => {
    const privateKey = crypto.createPrivateKey(wallet.credentials.privateKey);

    return signers.newPrivateKeySigner(privateKey);
}

Originally posted by @jeffbicca in hyperledger/fabric#4358 (comment)