What @knebb said 
You can read more about the requirements for running a UCS domain here:
https://help.univention.com/t/requirements-for-a-successful-join-into-the-ucs-domain/8842
In general all UCS systems that contain their own LDAP server (DC Master, DC Backup, DC Slave) should have themselves set as their nameserver1, not an external machine.
m.