Warning: This post contains a solution!
Only continue if:
1.) you want to see a possible alternative solution or
2.) you are stuck and need a hint!
Login using given credentials.
The message on the page is:
Access disallowed. You are not logged in
Intercepting the request using Burp shows, that there is a cookie value called
loggedin=0. Changing this value to
1 and repeating the request is enough to solve this level.
Access granted. The password for natas6 is aGoY4q2Dc6MgDq4oL4YtoKtyAg9PeHa1