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I had the same problem, don't know exactly anymore what went wrong, also had problems afterwards because nobody could actually sign the book without filling in ALL the fields, so I had to fill in defaults in the PHP post file.
There are some things you can do :
Look carefully to the file/directory permissions.
Update the template (in admin preferences general)
Turn PHP debug mode on (in admin preferences general)
I did not find any other guestbook module that is better.(except extremUSERguestbook, but that's only when you want your users, including you of course, to have their own guestbook)
Aloha