DCPROMO on Windows 2008 R2 64 bit fails

Question:
I have a Windows 2003 Domain with 3 dc’’s.
1 DC with Windows 2003 std. SP2
2 DC’s with Windows 2008 std. 64 bit.
I have run adprep from the Windows 2008 std. 64 bit image with /forestprep, domainprep and rodcprep with success.
I have now installed a Windows 2008 std. 64 bit R2 and added the server to [...]