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 membershi
p cache
02.12.2018 06:25:10,177 LDAP (PROCESS): Internal group membership cache w
as created
02.12.2018 06:25:10,184 LDAP (PROCESS): Using WHC as AD Netbios domain na
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_change
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 resul
File “/usr/lib/python2.7/dist-packages/ldap/ldapobject.py”, line 521, in resul
ldap_result = self._ldap_call(self._l.result4,msgid,all,timeout,add_ctrls,ad
File “/usr/lib/python2.7/dist-packages/ldap/ldapobject.py”, line 106, in _ldap
result = func(*args,**kwargs)
UNAVAILABLE: {‘desc’: ‘Server is unavailable’}