Ahhh. I've seen that before when I join a new node to a cluster, then the browser window that's still open on the node (to do the joining) then throws the same "proxmox VE ticket" error.
I think it just means the browser session is no longer valid for that node, in that window. If I then close that browser window completely then open a new browser window to visit the cluster ui again, it's always then worked.
So... I guess you could close all of your browser windows to the Proxmox server(s), then re-open a single new one and see if that fixes things?