A couple of things to check:

1. The site's /admin/includes/configure.php (where admin is the name of your admin directory). Make sure that the HTTP_SERVER value doesn't have a www. prefix.
2. Check the site's root .htaccess file to see if it's got some kind of redirection going on.
3. Check the site's /logs directory to see if any myDEBUG-adm-*.log files are present.