I am not an expert on the network part but I can help on the NBD part
We added some info in this PR https://github.com/vatesfr/xen-orchestra/pull/6596, it should be merged in this form or another in master in a few days. It will give the user more info on the NBD usage and the cause (misconfiguration, or network error)
Then if XO ( or the proxy) can't connect to the host through the NBD (for example with a network filtering/ routing), it will fall back to the reliable (but non parallelizable) api call
Finally , the backup done with NBD or the legacy api call will produce exactly the same data so they are compatibles, but using the same target (as in the same directory , same remote and same VMs) for multiple job can lead to complex support tickets with interesting race conditions.
I would advise to only use the same remote/directory only if the backups jobs does not concern the same VMs
opened this pull request in vatesfr/xen-orchestra
feat(@xen-orchestra/backups): more information on nbd backups