The appliance has sent a message, that the Cluster is in a Split Brain Situation.
A split brain in Clusters occurs, if for example the heartbeat connection is unavailable and therefore both nodes try to takeover the services.
In this state, both nodes may be either both secondary or primary and the Cluster itself failes.
The split brain state can be resolved using the Cluster Management or Split Brain Recovery Diagnose.
If the Problem cannot be solved this way, the cluster needs to be setup from scratch
It is highly recommended to use a direct physical Connection via cross over cable for the heartbeat.
Using a Switch instead could cause this Error, if a maintenance on the switch is performed.
If however a Switch needs to be used for the heartbeat, the passive node should be shutdown, before a maintanence on the Switch is done.