iThemes Security is one of my favorite security plugins, but it can be a little buggy. I am always very careful about upgrading the plugin because sometimes it locks me out of the site by redirecting me back to the home page whenever I try to login. When that happens, I can usually change the plugin file name through ftp/cpanel and then get back in. Then I can reset the plugin file name and everything works fine.
However, the last time I upgraded, every time I changed the name back to what is should be, it kept sending me to the home page and I couldn’t change any options. Luckily, I found this post on WordPress.org by davidebabylonia
In your cpanel/ftp folder, go to wp-content/uploads/ithemes-security, there is one file in there named isec_away.config. Download that file anywhere and then delete it from the server.
Then, open the dashboard of wordpress again. It should be available to you now, although you may have to refresh. Open SECURITY from the left hand menu. Once on the dashboard of the plugin, scroll visually down to the section “Security status” and click on the “completed” tab.
On the line where it says away mode, click edit and uncheck the thickbox. DO NOT press save yet.
Return to the ftp program and upload again the isec_away.config in the original position. Go back to wordpress and press save.
This fixed my problem and saved the day! Thank you to davidebabylonia! You can see the full discussion here: http://wordpress.org/support/topic/cannot-log-in-to-wp-admin-4?replies=82