Delete the Cluster from a node (resetting the cluster)

Do this in SSH, not in the shell via webgui.

(this worked from 6-8.1 tested)

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systemctl stop pve-cluster corosync
pmxcfs -l
rm /etc/corosync/*
rm /etc/pve/corosync.conf
killall pmxcfs
systemctl start pve-cluster

#wait 1-2 min. 

systemctl start pve-cluster.service
systemctl start pvestatd.service
systemctl start pvedaemon.service

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Ghost Nodes

if you are keeping one of your hosts, you will also have to remove data about previous nodes that will still show in your webgui. 

Go into  /etc/pve/nodes/  and rm -rf the folder with the name of the node that is no longer part of the cluster. This will remove the ghost nodes from your GUI.