Logging out in XMB requires the destruction of a cookie. There's only a few lines of code in the whole source code base which can do this, and it's not in the post.php where you are saying you're logging out.
Do you have ANY cookie blocking software, or does your browser do this?
Try
a) checking if you have any spy ware installed
b) removing or altering your cookie blocking software to allow AVDS to set cookies
c) try logging in using the "Secure" option here:
http://www.aussieveedubbers.com/forum/misc.php?action=login
and see if it persists. If it does, your browser or cookie blocking software is forcing you out.
Also, the cookies have expiry dates of approximately 1 week. Adjust your time zone in the User Control Panel to be the correct local time. Please check to see if you have the correct time (including year).
thanks,
Andrew