I always have the same trouble using that plugin.
When I create a New USER, it works fine and I receive an email with a password.
So I try to log in on form "Existing Users Login", but it always refuses with such message.
You entered an invalid username or password.
I can not log in but going on wp-admin page.
If I log on wp-admin page one time, then I can log in using the WP Members login page...
thank you for any help.