In the mean time I have tried to reset Update manager database as per article below. However, the problem still persists.
Also, I have observed that when I test connection, it says some url's are not accessible. (https://www.vmware.com) However when it try to redirect it from one of the machines that have internet access to use it as proxy, the download source still remains disconnected.
VCSA 6.5 - Update Manager not work
Is this something related with Certificates or firewall ports.. how do I check?