Hi Francesco,
do you always have the same error?
Yes
Are you by any chance using the post copy migration mode?
Not sure what that is. The migration is initiated by either putting the host in maintenance, or selecting 'upgrade' from the menu.
Could you please share the libvirt debug logs, at least on the source side?
https://wiki.libvirt.org/page/DebugLogs
You may want to do a test run with the debug logs turned on and disable them just after, those are VERY verbose.
Before I put it into debug mode, I'd tried to migrate the troublesome VM by itself, and it worked. So I then thought I try putting the host that has that VM into maintenance, and it successfully copied that VM over, but failed on another VM this time (a Linux VM). I took the host out of maintenance, and switched on debug mode (restarting libvirtd made the host rather unhappy, but it sorted itself out), and then tried putting the host back into maintenance. It complained about another VM failing to be migrated, though I couldn't see that VM on the host. It did succeed in moving the VM that was causing it problems, and is now in maintenance mode. So it seems a bit hit and miss. Do you still want the logs? Thanks, Cam