Hi,
I usually use KVM and VMWARE as hypervisor so I’ve no experience with Xen and VNC but maybe you can check this post (https://forum.opennebula.io/t/vnc-novnc-ready-native-websockets-canvas-rendering-error/3022). In that post I try to explain some things you can check just to be sure that basic VNC settings are there (VNC proxy is running, port is listening, firewall is not blocking connections… so on).
Try to read it and if you need any help we’ll try to help, and as Thomas says maybe logs in /var/log/one can give you more info.
Cheers!