Why if i try to login with empty password i receive the message "Failed to authenticate: Access Denied" or "Failed to authenticate: User is blacklisted", Joomla correct message is "Empty password not allowed"?
You have some other component that do this message.
Still in module and component for login you have default value for Username. Joomla use the placeholder text, probably you have template override for com_users and mod_login.
if you want we would like to investigate, if you want you can post your site details (you can use the Site Details tab below the reply).
Ok,
we have solved your problem.
for some strange reason when you click on "Login with Facebook" the form is submitted with your Username and Password empty.
This happens because the buttons of the "Social Connect" are in the form to login.
Normally this should not happen because we have incorporated the Directive preventDefault on the buttons.
Solution:
We removed the tags { social connect } from login form. And we added the tag in another module(custom HTML) same in the "login" position after login module.
