Weird. With you saying admin.pl I would of said 1.5 as 2.2 uses PHP. I believe your problem lies in the LAST_VISIT field of the book_auth table. This problem is usualy associated with updating from 2.2 to 2.3.x and not updating the guestbook. Do you have access to phpMyAdmin?
if everything else is working fine simply download reset.zip and upload the file it contains to your guestbooks folder. Then follow the instructions for it.