User's Guestbook don't wotk properly when drupal is installed in a subdirectory

I have my site installed in var/www/drupal/ and the url's to the user's guestbooks are returned as followed:

http://vliestroompje.homeip.net/guestbook/2

The site guestbook has a correct URL returned: http://vliestroompje.homeip.net/drupal/guestbook/0

Although the URL's don't work properly, see my previous bug report for this error.