Looks like you are probably right. I just checked permissions and all were off except Admin access for Admins.
I will ask the users who are having problems to try again.
I believe there is a problem with cookies that may be related. I mostly use Firefox on a Mac and sometimes when I go to PP CLassifieds admin I have to log in three times.
I am already shown as logged in, I click admin - it takes me to log-in, I log-in again and it takes me to the user index, I click Admin and log-in again - it takes me to the user index. Usually I get to the Admin page on the third attempt. Sometimes I click admin and i get to the admin page immediately. I don't normally log-out so I am guessing there may be a cookie problem.