needorf, you are right on, the adminpages variable seems to be set wrong. For those who might not understand what to do from needorf's post, here's what you gotta do:
1. go to your config file:
<your_document_root>/phplist-2.10.5/public_html/lists/config
2. find that $adminpages line and set it to what your page is on your site:
Example:
http://www.example.com/whatever/phplist/...
Set:
$adminpages = '/whatever/phplist/...';