Hi,
I just tried with singe server backups - seems to work a bit better. Nevertheless I often get (this one is a windows machine):
INFO: 99% (65.2 GiB of 65.8 GiB) in 30m 51s, read: 44.0 MiB/s, write: 27.8 MiB/s
ERROR: VM 762 qmp command 'query-backup' failed - got timeout
INFO: aborting...