assistance with guestbook v1.51 error

I have been trying to get the guestbook to work. Initially it was working but after the number of entries exceeded 20, I began to get the following error:

Guestbook - Error
Cannot add entry to guestbook because the html tag

<option value="guestbook.html" selected>Guestbook</option>
was not found in some guestbook files.

I am not familiar with perl and would appreciate any assistance on getting this guestbook to work for me.

thanks in advance for any help in solving this problem

gerry