Automatic confirmation emails to people who register contain unformed usernames. Example, instead of the username entered in the registration form (lets say, John) one contained:
Those inaccurate usernames do not work for logging in to our site, although if a user can remember the username they entered in the registration form, they can use it along with the password they entered (also returned by the plugin) to log in.
The plugin worked properly at one time, but people are complaining now. BTW, I am not a developer — and I didn’t know how to answer the question about setting the plugin to inactive, because that would completely disable the function.
I am sorry to tell you that “Register Plus Redux” is not my plugin. I only wrote the add-on to download the users data: http://wordpress.org/extend/plugins/register-plus-redux-export-users/ called Register Plus Redux export users.