AR Guestbook - "Oops! Page not found!"

[d-l-m-k]

here is my problem:
I've implemented the plugin arguestbook into my website.
Some month ago everything was okay but now,
When i'm trying post something in the guestbook there is a page fault message:

"Oops! Page not found!
We are sorry, but the page you are looking for does not exist."

What can I do.
I searched for a resolution by myself, but I didn't get any.
I noticed, that the posts I made are saved into the 404.txt, not into the guestbook.txt file.

Do anybody know how I can solve this problem?
I would be very glad!