Can't join UCS-domain with member-server (solved)

I can’t join with the following error:
“Unter der Adresse des DNS-Servers konnte kein Domänencontroller gefunden werden. Die Netzwerkeinstellungen sollten überprüft werden.”

“No domain controller could be found using the address of the DNS server. The network settings should be checked”

what can I do to debug?

some additional info:
I have 1 UCS-Server with a Domain and he’s the only DNS.
I have successfully joined a Windows-PC to the UCS-domain.

I have no clue why… but now it worked.