Login attempts are logged in the
backend_user_throttle table. If a user with a given IP has too many failed login attempts over a certain timespan, logins for this IP will be blocked.
To resolve this issue simply find the corresponding entry in the
backend_user_throttle table and delete it. If you don't know the user's IP it is safe to truncate the whole table.
The same method works for the
RainLab.User plugin as well. Simply remove the corresponding entry from the