I have exactly the same problem.
I dived into the code and compared with the API test code provided in the dashboard.
It seems that there is a new problem with the API version in the wordpress plugin.
I tried adding (I do not recommend doing this on your site, it was just for a test)
$body’audience’] = “https://your_domain/api/v2/”;
in the get_token function of WP_Auth0_Api_Client.php
and I could validate my client id and client secret in the wordpress plugin …
It makes me think there is a now problem with the API V1 and the wordpress plugin …