Securitycheck Web Firewall plugin is enabled by default.
If for some reason that you don´t understand Securitycheck is blocking the access to your backend, you can disable the plugin running this query in the Mysql database:
UPDATE `#__extensions` SET `enabled` = 0 WHERE `name` = 'System - Securitycheck'
You must replace "#_" with your DB prefix.
With this action you should be able to login in the backend again. Please, contact me in this case to check what is happening.
If you are looking for common issues and the way to disable the plugin in Pro version, then you may have a look to this