Natas5 - natas6
Access disallowed. You are not logged in
My first thought was to check the cookies (Seen challenges like this before ;-]).
Used the cookie tab in firebug and saw it there, the value 'loggedin' set to 0.
Changed to a 1 and refreshed the page to get the following:
Access granted. The password for natas6 is aGoY4q2Dc6MgDq4oL4YtoKtyAg9PeHa1