Custom Service with OAuth2 and client credentials

Custom Service with OAuth2 and client credentials

We are trying to add a custom service in Zoho Billing (Subscriptions). The custom services uses OAuth2 with client credentials. The setup in the Zoho UI is easy, but when we create and connect a connection for the service we only get "Connection Authentication failed" as response.

The only information I get from the developer console of the browser is the following JSON response.

{
  "code": 1797,
  "isValidDomain": true,
  "isAuthError": true,
  "message": "Connection Authentication failed",
  "status": "failure"
}

The same settings work when I use them with postman.