Testing the plugin with Postman, with a very basic setup, I'm always getting "invalid_credentials" as response for the login request. The user can log in by means of the browser, with no issues, so the credentials are not the problem. I'm using the build 3.1.25 of October CMS.

Your help will be greatly appreciated.

The default parameters "email" and "password" are not being used by default. I solved my problem by explicitly adding those fields, in the backend plugin configuration.