Everything listed above is all part of the solution. Lab Manager 3.0.2.56
All hosts in my cluster had the "Agent Not Responding" error message
I could disable & re-enable the hosts all day, Nothing changes.
Re-installing the agent didn't work for me
I'll simply add I restarted the VMWare SLA Monitor service on the Lab Manager server itself.
All agents were responding properly on all hosts in the cluster after the services recovered.
Be patient when restarting the services & give everything a chance to catch up.
Hope this helps someone