First you should turn off displaying errors (for security) by adding this to your wp-config.php file.
Uh oh, I wonder why my code didn't get executed when you activated the plugin, which fixed this issue by first dropping the post_related index and then creating a new one upon plugin activation. So somehow that didn't happen. Try deactivating and then reactivating the plugin.
I wonder if someone could take a look at the create_function code and see if I'm doing something wrong.
here's the SQL that is supposed to have been executed...
ALTER TABLE $wpdb->posts DROP INDEX post_related, ADD FULLTEXT post_related (post_name,post_content);
gSearchGoogleGuess.setUserDefinedLabel("Googles Best Guess");
Replace with whatever you want..