React native PKCE expo authorization troubles

I have been trying to set up the PKCE authorization with an expo react native application. This went well, up until I tried to fetch the access token from the /oauth/token endpoint.

The relevant code:

import * as AuthSession from 'expo-auth-session';
import { pkceChallenge } from 'react-native-pkce-challenge';

const authorizationEndpoint = "https://{myurl}/authorize";
const accessTokenUrl = "https://{myurl}/oauth/token";
const {codeChallenge, codeVerifier} = pkceChallenge();
const redirectUri = AuthSession.makeRedirectUri();
  const [request, result, promptAsync] = AuthSession.useAuthRequest(
    {
      redirectUri,
      clientId: auth0ClientId,
      codeChallenge: encodeURIComponent(codeChallenge),
      codeChallengeMethod: AuthSession.CodeChallengeMethod.S256,
      responseType: 'code',
      scopes: ['openid'],
      extraParams: {
        audience: "{myaudience}"
      }

    },
    { authorizationEndpoint }
  );

Here I got the response:

{
  "authentication": null,
  "error": null,
  "errorCode": null,
  "params": Object {
    "code": "f7SPGa4iMvQUBnBI",
    "state": "lCVrP2kake",
  },
  "type": "success",
  "url": "{redirectURI}?code=f7SPGa4iMvQUCnAI&state=lCVrP2kaka",
}

fetch(accessTokenUrl, {
          method: "POST",
          headers: { "content-type": 'application/x-www-form-urlencoded' },
          body: JSON.stringify({
            code: encodeURIComponent(result.params.code),
            redirect_uri: encodeURIComponent(redirectUri),
            client_id: encodeURIComponent(auth0ClientId),
            grant_type: encodeURIComponent('authorization_code'),
            code_verifier: encodeURIComponent(codeVerifier)
          })

To this I got a 401 response:

 "error": "access_denied",
 "error_description": "Unauthorized",

Hi @Espenbfo,

Welcome to the Auth0 Community!

I understand that you have been encountering errors trying to exchange for an access token with your React Native App.

First, could you please clarify whether you are using one of our SDKs?

Next, have you taken a look at your Auth0 Dashboard Logs to see more details regarding the 401 Unauthorized error?

Could you please relay the error details to me to further assist you?

Looking forward to your response.

Thank you.