"Invalid client auth token." when requesting a visual studio token

I am new to oauth 2.0 and trying to get a token to access Azure DevOps Services REST Api but I keep getting “Invalid client auth token.”.
I created an app and can successfully obtain an authorization_code for it from the https://app.vssps.visualstudio.com/oauth2/authorize endpoint, both through Postman and with the full URL in a browser window, but the https://app.vssps.visualstudio.com/oauth2/token endpoint call fails and the Postman Console shows the message:

  • Error:“invalid_client”
  • ErrorDescription:“Invalid client auth token.”
    I did not find help on google for this, has anybody already come across and possibly solved this issue?
    Many thanks in advance.