Unable to sync all the AD users to UCS



I just installed UCS and able to join my AD Domain using the AD connector. However, computer objects and groups able to sync to the UCS. Only a few AD users can be synced to UCS. Appreciate any help for fixing this issues. Below is the UCS version number and the log.
UCS Version
4.3-2 errata344 (Neustadt)
UMC Version


02.12.2018 06:25:09,711 MAIN        (------ ): DEBUG_INIT
02.12.2018 06:25:09,990 LDAP        (PROCESS): Building internal group membership cache
02.12.2018 06:25:10,177 LDAP        (PROCESS): Internal group membership cache was created
02.12.2018 06:25:10,184 LDAP        (PROCESS): Using WHC as AD Netbios domain name
02.12.2018 13:43:42,599 LDAP        (PROCESS): sync to ucs:   [windowscomputer]
[    modify] cn=whc-dt-17097,cn=computers,dc=whc,dc=org,dc=hk
02.12.2018 16:25:11,312 LDAP        (WARNING): Exception during search_ad_changes
02.12.2018 16:25:11,313 LDAP        (WARNING): Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.7/univention/connector/ad/__init__.py", line 2283, in poll
    changes = self.__search_ad_changes(show_deleted=show_deleted)
  File "/usr/lib/pymodules/python2.7/univention/connector/ad/__init__.py", line 1299, in __search_ad_changes
    returnObjects = search_ad_changes_by_attribute('uSNCreated', lastUSN + 1)
  File "/usr/lib/pymodules/python2.7/univention/connector/ad/__init__.py", line 1290, in search_ad_changes_by_attribute
    return self.__search_ad(filter=usnFilter, show_deleted=show_deleted)
  File "/usr/lib/pymodules/python2.7/univention/connector/ad/__init__.py", line 1247, in __search_ad
    rtype, rdata, rmsgid, serverctrls = self.lo_ad.lo.result3(msgid)
  File "/usr/lib/python2.7/dist-packages/ldap/ldapobject.py", line 514, in result3
  File "/usr/lib/python2.7/dist-packages/ldap/ldapobject.py", line 521, in result4
    ldap_result = self._ldap_call(self._l.result4,msgid,all,timeout,add_ctrls,add_intermediates,add_extop)
  File "/usr/lib/python2.7/dist-packages/ldap/ldapobject.py", line 106, in _ldap_call
    result = func(*args,**kwargs)
UNAVAILABLE: {'desc': 'Server is unavailable'}