We had an issue where a new hosted was added but we could never access the VNC's of it. If we did we got an authentication failure. This is a simple fix. Proxmox uses a language locale to pass the authentication along via SSH. "LC_PVE_TICKET". However this is disabled in SSHD.
To enable it simply edit the config file at "/etc/ssh/sshd_config". Find the line "#AcceptEnv LANG LC_*" and uncomment it. Restart SSHD and it should work now.