OAuth2 authentication with client_credentials grant fails with HTTP 520

[razvanz]

I'm trying to obtain an access token using the documented OAuth2 client_credentials grant:

import { TrustpilotApi } from "trustpilot";

;(async () => {
	const client = await new TrustpilotApi({
		key: '<api_key>',
		secret: '<api_secret>',
		grantType: 'client_credentials',
		baseUrl: 'https://api.trustpilot.com'
	}).authenticate();

	const res = await client.get('/v1/business-units/all');

	console.log(res.data);
})()

However, authentication fails with:

    status: 520,             
    statusText: 'Unknown error',                                                      
    headers: Object [AxiosHeaders] {
      'content-type': 'text/plain',     
      'content-length': '0',    
      connection: 'close',      
      date: 'Tue, 20 Aug 2024 13:02:05 GMT',
      'x-cache': 'Error from cloudfront',
      via: '1.1 9aac88bd0767f985559187c36e716276.cloudfront.net (CloudFront)',
      'x-amz-cf-pop': 'CHP50-C2',                                                     
      'x-amz-cf-id': 'iX0LXSapKKvz7glJYMw9LrevY8t4kOTugsUdn1GtQavCzpVGSeE0IQ=='
    },                                
    config: {                                                                         
      transitional: [Object],       
      adapter: [Array],
      transformRequest: [Array],
      transformResponse: [Array],                                                                                                                                            
      timeout: 0,                                                                     
      xsrfCookieName: 'XSRF-TOKEN',                                                                                                                                          
      xsrfHeaderName: 'X-XSRF-TOKEN',
      maxContentLength: -1,
      maxBodyLength: -1,
      env: [Object],
      validateStatus: [Function: validateStatus],
      headers: [Object [AxiosHeaders]],
      auth: [Object],
      baseURL: 'https://api.trustpilot.com',
      method: 'post',
      url: '/v1/oauth/oauth-business-users-for-applications/accesstoken',
      data: 'grant_type=client_credentials'
    },

If I supply invalid credentials I get a different error, HTTP 401, so it's most likely a different error.