I was just reading some of the comments posted today and Marc Sevigny, one of the vSphere HA developers, pointed out something which I did not know. I figured this is probably something that many are not aware of so I copied and pasted his comment:
Another thing to check if you experience this error is to see if you have jumbo frames enabled on the management network, since this interferes with HA communication. This is document here in a note: http://kb.vmware.com/kb/2006729 To make it crystal clear: disable jumbo frames on your management network with vSphere 5.0 as there’s a problem with it! This problem is currently being investigated by the HA engineering team and will hopefully be resolved.
<Update> Just received an email that all the cases where we thought vSphere HA issues were caused by Jumbo Frames being enabled were actually caused by the fact that it was not configured correctly end-to-end. Please validate Jumbo Frame configuration on all levels when configuring. (Physical Switches, vSwitch, Portgroup, VMkernel etc)</Update>