I had the same problem with the recent iThemes Security upgrade as everybody else – I was unable to log into my admin area until I disabled the plugin by FTP.
When I got back into my admin area, I deleted the plugin altogether, then I deleted wp-content/uploads/ithemes-security/ by FTP. Then I opened PHPMyAdmin and removed the iThemes tables from my database as well as the iThemes rows out of my Options table.
Everything's back to normal except one thing. When I click 'Posts' or 'All Posts' in my admin area, I get redirected to http://mysite.com/wp-login.php?redirect_to=http%3A%2F%2Fwww.mysite.com%2Fwp-admin%2Fedit.php&reauth=1 – straight back to the login page.
Has anyone else encountered the same issue? Anyone fixed it? I've looked through all the .htaccess files in my site and I can't find anything that I think would cause this.