DBS API Server Error 500 on Sandbox calls - node.js

Question

I was just experimenting with the DBS API Sandbox here (https://www.dbs.com/developers/). We have to use OAuth2 to get the access token before passing it in the header to authenticate API calls. When I tested everything out on their test client UI I got the results back from the API call.
However, when I coded my simple node app, I could successfully perform OAuth2 and get an access token, but when passed in together with the clientId, got a server error 500 from the API.
For example, the GET call to get a bank account details:
const options = {
method: 'GET',
uri: dbsHost + rootPath + '/accounts/22909292550739201999095',
headers: {
'Content-Type': 'application/json',
'accessToken': accessToken,
'clientId': clientId
}
}
request(options, (error, response, body) => {
// Server error 500
console.log(response)
})
These were the exact instructions as those given on the test client UI. On closer inspection I realised the access token I always got back had 488 characters, but that of the test client UI only had 475. Any reason for this difference? How do I know OAuth2 was performed correctly and I'm getting the right access token?
Here's how I did OAuth2:
On the frontend:
let authorizationUri = dbsHost + rootPath + '/oauth/authorize'
authorizationUri += '?client_id=' + clientId
authorizationUri += '&redirect_uri=' + redirectUri
authorizationUri += '&scope=Read'
authorizationUri += '&response_type=code'
authorizationUri += '&state=0399'
windows.location.assign(authorizationUri)
And at the redirect_uri endpoint once I get back the authorization code:
app.get('/authCallback', (req, res) => {
let authCode = req.query.code
// Encode client_id:client_secret in base-64 to pass in as Auth Basic Header
let b64 = new Buffer(clientId + ':' + clientSecret).toString('base64')
const options = {
method: 'POST',
url: dbsHost + rootPath + '/oauth/tokens',
headers: {
'Authorization': 'Basic ' + b64
},
form: {
'code': authCode,
'redirect_uri': redirectUri,
'grant_type': 'token',
'state': '0399'
}
}
request(options, (error, response, body) => {
if (error) {
console.error('Access Token Error', error.message)
return res.status(500).json('Authentication failed');
}
let json = JSON.parse(body)
let accessToken = json['access_token']
// Make a call with this access token
})
})