Last night I tried upgrading to VCenter 5.1 and had problems after the install where I could not log in. Being late, I failed back to a snapshot version of my VCenter server taken just prior to the upgrade. After I was able to back into VCenter all my hosts, clusters and VM's were available. But my clusters are all erroring with VCenter Server unable to find a master VShpere HA agent in cluster. Each host under the cluster has an error: VSphere HA agent for the host has an error: The VShpere HA agent is not reachable from VCenter Server.
Under vCenter advanced settings for SSL, if I check the vCenter requires verified host SSL certs, there are no servers showing up in hosts window.
Being a production environment, I'm hesitant to remove the hosts from vcenter, even in disconnect mode, without some assurance that I'm not going to lose my hosts and I'm going to lose my guests or not be able to bring the host back into an HA environment.