Hi @VascoToken! It would be helpful to let us know when exactly you hit that error, for what request, and at what point.
I am not familiar with Forgerock, but I can tell you the steps to authenticate a user with your app, maybe it helps:
1) you make a request to oauth/request_token passing in the callback URL in the oauth_callback parameter
2) you save in the session the tokens you receive
3) you redirect the user to oauth/authorize passing in the variable oauth_token that you just received
4) if the user gives their permission, you need to convert the request token to a usable access token by making a request to oauth/access_token passing in the parameters oauth_verifier and
oauth_token that you have just received in the url, after the user was redirected back.
Maybe this helps you figure things out with Forgerock.
All the best!