Even if an IP address is entered for Login Server on Cisco Jabber, if CUPS Node name (CUPS Admin Page -> System -> Cluster Topology) is configured as HOSTNAME rather than IP Address and that Host name is not resolvable, Cisco Jabber will be unable to connect to that server. Notice in the logs where even if we had entered IP address of CUPS, Cisco Jabber will try to connect to the Hostname:
Have DNS Entry to resolve CUPS FQDN and also make sure the workstation is part of the AD domain. If workstation is not part of the Domain, the Hostname will not be resolvable, only the FQDN would be, in that case we will have to either make that workstation part of the domain or create a local host entry on that workstation by editing the "hosts" file located in C:\WINDOWS\system32\drivers\etc and then log back into Cisco Jabber.
When DNS server has an entry for CUPS FQDN, make sure that the workstations, where Cisco Jabber for Windows is deployed on, are part of the domain.