By default Spring OAuth requires basic HTTP authentication. If you want to switch it off with Java based configuration, you have to allow form authentication for clients like this:
@Configuration
@EnableAuthorizationServer
protected static class OAuth2Config extends AuthorizationServerConfigurerAdapter {
@Override
public void configure(AuthorizationServerSecurityConfigurer oauthServer) throws Exception {
oauthServer.allowFormAuthenticationForClients();
}
}