The error indicates a problem with the PHP script in admin.php. Simply replace admin.php with the one from your version, which is the latest stable available from the member area, v1.87d. To make sure everything is correct I have, in fact, overwritten all of the ListMail files on your site with the ones from the zip. (this is 100% safe to do because config.php is named config.php.txt in the ZIP)
Regards
