Problem with proxmox GUI and cluster management.
From one node of the cluster I saw on /var/log/syslog file that
Apr 12 16:34:37 t1 pveproxy: proxy detected vanished client connection
and when we access to the Web GUI of that node, Show Node ‘t1’ is down.
The solution is to simulate a reboot of all the machines without rebooting the VM on them.
So this is the sequence to launch on every node of the cluster, without some particular order
/etc/init.d/cman restart /etc/init.d/pvedaemon restart /etc/init.d/pvestatd restart /etc/init.d/pve-manager restart /etc/init.d/pve-cluster restart
Now, it works great.