Jason,
I am not sure your advice is the correct implementation. Removing the check will allow staff without admin privileges to possibly access admin pages! It is better to get to the root cause of the problem.
I am also not sure the condition you mentioned will result in access denied. staff.inc.php does the login checks and the only way to get the error as you described will be when you are already locked in as staff (non-admin) and try to access admin page.
Regardless of the issue at play, you might want to change the code as shown below;
if(!$thisuser or !$thisuser->isadmin()) {
header('Location: index.php');
require('index.php'); //Just in case of of header sent already
exit;
}I am very interested in replicating the error as you described. Feel free to PM or email me.