[Samba] samba ad sysrepl

[basti]

Hello,
I have installed an samba ad1 and an samba ad2 with replication.
On the dc1 "samba-tool drs showrepl" say "... was successful"
On dc2 there is the same.

When dc1 goes down I get an Error (on my client):

Active Directory Users and Computers error: "server is not operational"

What is wrong here?
For my understand that is no Replication, when one goes down and the
management console doest work anymore.

Best regards