[Plugin: iThemes Security] Registration URL not working

I’m in the process of setting up a blog at the company I work for, and the theme was installed and configured by the web editing team, who have also installed iThemes Security. When they did the theme they put the wrong registration URL in the theme settings, and now when I try to use the default one I just get a garbled page of code.

The login page is http://blog.[domain].com/loginxx but I’ve tried using http://blog.[domain].com/wp-login.php?action=register and http://blog.[domain].com/loginxx.php?action=register and neither work. I don’t want to go messing around in the theme’s code, I just want people to be able to register on the blog!

(btw, loginxx isn’t the real slug!)

What should I do?