Already other people had similar problems, but I didn't found the exact error yet. Perhaps you can help me to identify the problem.
Can you give me some more information to localize the problem:
1. Check if the shortcode is working:
If you disable the plugin the shortcode text "[comment-guestbook]" should be visible on the site. If you enable the plugin you shouldn't see the text "[comment-guestbook]" on your site because it will be replaced with other output (also when it is empty in your case). Can you confirm that?
2. Check if a php error occurs:
Please add additional text in the guestbook site before and after the shortcode. Is all the other text visible on the site, or is the text after the shortcode missing?
3. Check the settings page:
Did you change something in the settings (see Comments -> Guestbook)? If you change something do you find any changes on the guestbook site?
I hope you can answer the upper questions to help me identifying the problem.
Thanks in advance.