[Samba] Problems joining Samba4 domain

[X-Dimension]

Hi!

I've installed Samba4 like described here on Ubuntu 12.04.3

http://www.matrix44.net/cms/notes/gnulinux/samba-4-ad-domain-with-ubuntu-12-04

DNS with BIND_DLZ backend seems to work and i can add DNS records 
without a problem with Microsoft RSAT.

When i try kinit on the server i get the following:

root at PDC:~# kinit administrator
Password for administrator at MYDOMAIN.LAN:

-> Works fine

root at PDC:~# kinit administrator at MYDOMAIN.LAN
Password for administrator at MYDOMAIN.LAN:

-> Works fine

root at PDC:~# kinit administrator at mydomain.lan
Password for administrator at mydomain.lan:
kinit: KDC reply did not match expectations while getting initial 
credentials

-> Don't work

When i try to join a Windows 7 Professional client to my domain, i get this:

1. Joining as user "administrator" to domain "mydomain" or "MYDOMAIN" or 
"mydomain.lan" or "MYDOMAIN.LAN"
  -> wrong username or password

2.   Joining as user "administrator at mydomain" to domain "mydomain"
  -> works fine
But after a reboot i can't login as "administrator at mydomain" and i get 
wrong username and password

My krb5.conf looks like this:

[libdefaults]
         default_realm = MYDOMAIN.LAN
         dns_lookup_realm = false
         dns_lookup_kdc = true

What goes wrong here?

Thanks for help!