AGB 2.3.1 stops working after 1st post

For some reason my AGB stops working after the first post. I get the follwoing error message for index.php:
Query Error
--------------------------------------------------------------------------------

MySQL Error : Query Error
Error Number: 1 Can't create/write to file '/var/tmp/#sql11ece_11_0.MYI' (Errcode: 2)
Date : Mon, August 5, 2002 00:02:55
IP : 128.107.253.40
Browser : Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; Q312461)
Referer :
PHP Version : 4.1.2
OS : FreeBSD
Server : Apache/1.3.26 (Unix)

If I go into the admin and delete the last message posted (the 2nd one), every thing returns to normal until some tries to post again. I can't figure out why it will only allow one post. Any suggestions?

Please post the url where the guestook is installed

The URL is http://www.mayaaryanna.com/guestbook/index.php if you want to take a look. I reset the username and password to the defualt (test, 1234) if anyone wants to play around with it.