First, please make sure to always run the latest version!
We constantly release bug and security fixes.
Did you set redundant links for Corosync? [0]
Without those, once the network Corosync is running on fails, your hosts won't be able to reach the others in the cluster leading to them fencing themselves [1] if HA resources are defined on those hosts.
Can you provide your Corosync config?
cat /etc/pve/corosync.conf
The network config as well, please:
cat /etc/network/interfaces
And the output of the following command:
ha-manager status
[0]
https://pve.proxmox.com/pve-docs-7/pve-admin-guide.html#pvecm_redundancy
[1]
https://pve.proxmox.com/pve-docs-7/pve-admin-guide.html#ha_manager_fencing