Hey! Well it really isn't a hacker but it IS a spammer... they have "spiders" that crawl the internet looking for files named "guestbook.php". The reason for the error is the spider is generic so it will work on any software and it isn't accessing the file correctly. If you check in your database under the guestbook table you might be shocked, mine was full of ads for Viagra and other nasty stuff. There are a two solutions (I did #1):

1. Go in admin under menu builder and remove see more guestbook from the profile menu, then delete guestbook.php ... why do we need a guestbook AND profile comments??

2. Rename guestbook.php to something else, like book.php and then go in the admin menu builder and change the link to whatever you named it.

They really aren't hurting anything because the comments don't show up on the profiles anyway, just in the database.. but those error messages are ANNOYING.