Trouble updating

I have a rather old version of this plugin and I’m trying to update to the current version. Whenever I try to make any changes to the plugin, however (update, activate, edit, delete), instead of performing the requested action I am redirected to the main page of my website. Any thoughts on how to fix this? I really appreciate any help you can give.

http://wordpress.org/extend/plugins/wordpress-23-related-posts-plugin/