It certainly appears to be something related to whmcs proxmox addon. Please try updating to the latest version and see if it helps.
Also, ensure that you are connected to proxmox nodes from whmcs >> Servers
Run the below command on every proxmox nodes in the cluster. (reinstalling vnc package)
apt-get install --reinstall novnc-pve
Then copy over the contents of folder "6.3" (in my case I am using proxmox 6.3) from "UploadOnProxmoxServer" (which came with your whmcs addon )to /usr/share/novnc-pve...