Administrator access denied on multisites

I have set up a multisite configuration on my webserver per the instructions found here:

Multi-site setup in 5.x using CPanel
http://drupal.org/node/125539

My sites directory structure is:
sites/example.com
sites/site1.example.com

Installation went fine and I was initially able to administer the site on the subdomain (site1.example.com). However, after logging out and trying to log back in I get "Access denied - You are not authorized to access this page". If I request a new password and follow the link from the subsequent email I am able to log back in, change my password, and administer the site ...only to be denied access once again after logging out and trying to log back in.

Log entries during this process are as follows:
access denied 03/19/2007 - 19:51 admin Visitor
user 03/19/2007 - 19:51 Session opened for Username. Username
user 03/19/2007 - 19:50 Session closed for Username. Username
user 03/19/2007 - 19:50 User Username used one-time login link at ... Visitor
user 03/19/2007 - 19:50 Password reset instructions mailed to ... Visitor

I found a post that suggested adding this to session.inc:
register_shutdown_function('session_write_close');

This did not work.

I have no problems logging in and out of my main site (example.com).

Multisites is key to my current project and would love to get this working as soon as possible.

I am using:
Drupal 5.1
PHP 5.1.6
MySQL 4.1.21
Apache 1.3.37 (Unix)
Drupal installed in public_html/
shared hosting environment

Thanks.