With this plugin activated, I cannot access the WP upgrade page. I get a message that says
You are not allowed to call this page directly.
Simply disabling this plugin allows me to upgrade as normal.
Clearly, I'm not the only one with this problem: http://wordpress.org/support/topic/385355?replies=7#post-1558251.
What's even more frustrating is that in order to report this serious bug to the author on his site, you must have a paid subscription to his website! Very poor form for a developer.