If you've recently changed your Proxmox cluster, you may find that one or more of the nodes is unable to view VNCProxy for it's VMs. In the error message you'll see something like this:
Connection closed by 10.100.0.20 port 58876 [preauth]
The solution here is pretty simple. We must update our keys for the cluster by running the following on each Proxmox server on the cluster.
To do so, go to each server, then to Shell (or ssh into each manually) and run:
pvecm updatecerts --force
And that's it! You should now be able to authenticate properly.