Win 2008 R2 hung during Join

Hi Everybody,

I have following installation

1 x Win 2003 R2 SP2 Std x64 (PDC)  - 1 x Win 2003 R2 SP2 Std x64 (Backup DC) - Several  Win 2008 R2 SP1 Std Servers

PROBLEM:
If i try to Join any of this Win 2008 R2 Servers as Member to the Domain i get following Situation
After entering the Admin Account and Password - Win 2008 Server shows Hourglass - but never returns
neither with a Error Message nor with the Welcome to Domain Message. The Window is still presented.
Does not Timeout  - still shows hourglass after 3hours.

Any Idea what goes wrong?