That machine has suffered a repeated hardware failure, and has been taken offline, and I think I'll reinstall XCP-ng from scratch on it, so the exact problem described here is moot.

However, I'm still interested to learn by what mechanism XOA connects to the VM's VNC console, if anybody feels like teaching me? Thanks in advance.

I had some kind of hardware lockup, thus a forced reboot (ctrl-syrq+s/u/s/b) and the end result was that /var/lib/xcp/state.db was corrupted and xapi was not starting. I ended deleting that file to have it automatically recreated empty/from scratch, and restoring all my VMs from backup.

Since then, all my Xen Orchestra installations cannot connect to the (VNC) console of the VMs running on this host; it works on the same XOA for another host that is in another pool, so the problem is probably not on XOA but on on the XCP-NG host.

I'm not sure by what mechanism XOA connects to VNC (how it connects to the Unix socket /var/run/xen/vnc-N from what is a remote machine), so if anybody has a hint how I can debug this?

Connecting to the VNC console by redirecting /var/run/xen/vnc-N (where N is the domid) to a TCP port and connecting to that TCP port still works, so the VNC console itself works.