the key is only for the admin
So we go back and try to login as admin. But we see a message that the admin login is disabled. We use burp to do closer look. We login again and we see a cookie named username with some hex numbers. After some tests, it seems that the cookie is the username xored with something. So we try to login with "admina" and use burp to delete the last two hex numbers of the cookie.
Voilà! We have the key.
who wants oatmeal rasin anyways twumpAdby