After patching our vCenter this afternoon from 8.0 Update 3d to 3e I encountered the following errors:
Cannot find vSphere HA master agent
vSphere HA agent for this host has an error: The vSphere HA agent is not reachable from vCenter Server
Broadcom has a fairly helpful guide which basically involves turn HA off and back on again.
However it doesn’t mention to wait at least 5 minutes after switching HA off. The first time I did the switch off and back on it didn’t fix the issue but after switching off, waiting 5 and then switching back on the hosts remediated and elected themselves correctly.