On various investigations using Wireshark I discovered that "Server Manager for Domains" uses the SAMR "LookupNames" call to find informations about a certain "SERVERS" object.
We don't find this in the SAM database and return "NT_STATUS_NONE_MAPPED". I think this is a special object which SAMBA 4 should provide.
Tested against Windows Server - same result.
Seems to be a bug in Server Manager for Domains - or maybe it has been in use by older Windows NT Server versions?