This is a little after the original discussion, however it may well help someone in the future…
In our case we experienced this issue after a Greenfield install of vSphere 5.1 & VSA 5.1.1.
On two of the three hosts (the hosts assigned for VSA) the summary showed the alert…
The vSphere HA agent on this host cannot reach some of the management network addresses of the other hosts….”
It then gave the FQDN of one of the hosts and an IPv6 Address
I had planned the install around IPv4 rather than IPv6 so figured that perhaps turning off IPv6 might cure the issue, as reviewing the switch setup I could also see an IPv6 network address for the Management Network that I hadn’t specified during the installation.
On each of the Hosts I went to the “Configuration” tab, “Networking“, “Properties“, “Edit Management Network“, “IP Settings“. I started by checking that I had setup an IPv4 address then ticked “No IPv6 Settings“.
On the HA cluster I then un-ticked “Turn on vSphere HA“, let it go through in the Recent Tasks pane, then turned it back on. This resolved the issue for us.