Acropolis Upgrade: Failing at PreUpgrade Check "Failed to connect to Hypervisor management server ip

Running DELL XC630 5 node cluster with ESXi 6.0 / NSX
Trying to use One Click Upgrade to move from NOS 4.5.2 to 4.6.2
PreUpgrade Check fails with "Failed to connect to Hypervisor managment IP X.X.X.1 Failed to connect to Hypervisor managment IP X.X.X.2 Failed to connect to Hypervisor managment IP X.X.X.3 Failed to connect to Hypervisor managment IP X.X.X.5"

x.x.x.1~5 being ESXi IP External addresses

- SSH to all ESXi from all CVM on both Internal and External IP works. (also the reverse connection is working)
- ESXi reboot / Genesis restart done
- fix_host_ssh done
- even updated all ESXi root password on CVM (even though unchanged just to be sure)
- No Alert/Warning under PRISM
- ncc health_checks run_all came out with flying colors.
- Identical setup at DR site having same issue/symptoms

tail -f genesis.out shows:
esx_utils.py:135 Connection to host x.x.x.1 failed x.x.x.1:443 is not a VIM server
host_upgrade_helper.py:655 Error in connecting to local hypervisor

Stumped and hence sending out SOS.
Would really appreciate your support / suggestions.