Can’t access backend (old WP installation)

Hey guys!!

I have a problem with one of my customers sites.
It is still 3.6.. i think that could be causing the problem. The person which managed the site before didn’t even loged in for the last years and now i have a completly outdated and broken site to work with..

The Problem:
After trying to login via /wp-admin i get a “Authentication Required” pop-up to login.. I think it is some kind of security plugin. After i try to login with my username and pw, the pop-up apears again and again. I don’t get a “bad passwort” or something. I am 100% sure my user and pw is correct, i also tried with a new user which i created via phpmyadmin. Both of them dosn’t work.
Another try with /wp-login… ofc i get the same pop-up which dosn’t work, but if i press “cancel” in the pop-up the usual login window apears, but without any css. After isert my user into the usual wordpress login form, i get the pop-up again which obviously dosn’t work…

But it gets even stranger. I deactivated all plugin (once via FTP and once via the database). After a retry the same window apears but with a little twist. It still doesn’t let me acess the Backend but i am able to login and see the frontend admin panel. After trying to press on any of the menu buttons, which should redirect me to the backend, i get the pop-up again and cant access the backend.

short:
I am able to login with my wordpress account, but cant access the backend. WordPress constantly wants login data via a pop-up window, which apears again and again, even with the correct user and pw.

– Even without plugins/theme the problem is still there
– I tried to copy it to a local installation.. same problem
– There isn’t any aditional user in the database, which could work
– I tried with all other related accounts ftp, mysql, host

Wordpress: 3.6
Theme: Avada 3.0.1

Plugins:
xhanch-my-advanced-settings (this one disables the frontend admin panel)
wordpress-seo
toggle-wpautop
guerrillas-facebook-like-box
disable-feeds
cachify
bj-lazy-load
allow-php-in-post-pages

I don’t know what to do anymore..

That you for youre help