An error may appear during VC upgrade stage 2.
Error occurred while trying to establish connection. Exact error: Problem communicating with the
vCenter Server Appliance given as source host or the ESXi host where the source vCenter is deployed
The connection from upgrading VC to source host or its hosting VC(the VC that manages source host) failed.
Please refer to the connection destination defined in Stage 1: the value you entered here represents that destination.
nslookup <destination-FQDN>