The resource owner or authorization server denied the request

Problem/Motivation

I can generate an access token using Postman but it raises an error in the Drupal logger as below:

League\OAuth2\Server\Exception\OAuthServerException: The resource owner or authorization server denied the request. in League\OAuth2\Server\Exception\OAuthServerException::accessDenied()

Symfony\Component\HttpKernel\Exception\HttpException: The JWT string must have two dots in 
Drupal\simple_oauth\Authentication\Provider\SimpleOauthAuthenticationProvider->authenticate()

I can navigate protected URLs using the same access token as well.