Remote backup fails: unsupported flags (0x2) in function qemuBackupBegin after update to 2.40

After update from 2.28 to 2.40 remote backup fails with: `Failed to start backup: [unsupported flags (0x2) in function qemuBackupBegin]` My hypervisor did not change on target machine. It is libvirt on ArchLinux: ``` libvirt-10.6.0 qemu-9.0.2 ``` I am not sure if this is revelant, but on backup machine libvirt and qemu was updated to: ``` libvirt 1:12.0.0-1 qemu-10.1.2 ``` Full log: ```Jan 17 00:20:01 serenity vm_backup.sh[300880]: [2026-01-17 00:20:01] INFO lib common - printVersion [main]: Version: 2.40 Arguments: /usr/bin/virtnbdbackup -U qemu+ssh://vm@universe/system --ssh-user vm -d rusty -o /backup/vm/rusty/2026-01 --noprogress> Jan 17 00:20:01 serenity vm_backup.sh[300880]: [2026-01-17 00:20:01] INFO root virtnbdbackup - main [main]: Backup level: [inc] Jan 17 00:20:01 serenity vm_backup.sh[300880]: [2026-01-17 00:20:01] INFO virt client - _connect [main]: Connected to remote host: [myhost.mydomain] Jan 17 00:20:01 serenity vm_backup.sh[300880]: [2026-01-17 00:20:01] INFO root virtnbdbackup - main [main]: Libvirt library version: [10006000] Jan 17 00:20:01 serenity vm_backup.sh[300880]: [2026-01-17 00:20:01] INFO root virtnbdbackup - main [main]: NBD library version: [1.24.0] Jan 17 00:20:01 serenity vm_backup.sh[300880]: [2026-01-17 00:20:01] INFO root disktype - Optical [main]: Skipping attached [cdrom] device: [sdb]. Jan 17 00:20:01 serenity vm_backup.sh[300880]: [2026-01-17 00:20:01] INFO root disktype - Optical [main]: Skipping attached [cdrom] device: [sdc]. Jan 17 00:20:01 serenity vm_backup.sh[300880]: [2026-01-17 00:20:01] INFO root virtnbdbackup - main [main]: Backup will save [1] attached disks. Jan 17 00:20:01 serenity vm_backup.sh[300880]: [2026-01-17 00:20:01] INFO root virtnbdbackup - main [main]: Concurrent backup processes: [1] Jan 17 00:20:02 serenity vm_backup.sh[300880]: [2026-01-17 00:20:02] INFO root checkpoint - redefine [main]: Loaded checkpoint list from: [/backup/vm/rusty/2026-01/checkpoints] Jan 17 00:20:02 serenity vm_backup.sh[300880]: [2026-01-17 00:20:02] INFO root checkpoint - create [main]: Next checkpoint id: [16]. Jan 17 00:20:02 serenity vm_backup.sh[300880]: [2026-01-17 00:20:02] INFO root checkpoint - create [main]: Parent checkpoint name [virtnbdbackup.15]. Jan 17 00:20:02 serenity vm_backup.sh[300880]: [2026-01-17 00:20:02] INFO root checkpoint - create [main]: Using checkpoint name: [virtnbdbackup.16]. Jan 17 00:20:02 serenity vm_backup.sh[300880]: [2026-01-17 00:20:02] INFO ssh client - connect [main]: Connecting remote system [myhost.mydomain] via ssh, username: [vm] Jan 17 00:20:02 serenity vm_backup.sh[300880]: [2026-01-17 00:20:02] INFO paramiko.transport transport - _log [Thread-1]: Connected (version 2.0, client OpenSSH_9.8) Jan 17 00:20:02 serenity vm_backup.sh[300880]: [2026-01-17 00:20:02] INFO paramiko.transport transport - _log [Thread-1]: Authentication (publickey) successful! Jan 17 00:20:02 serenity vm_backup.sh[300880]: [2026-01-17 00:20:02] INFO root virtnbdbackup - main [main]: Remote NBD Endpoint host: [universe.site.infoklinika.pl] Jan 17 00:20:02 serenity vm_backup.sh[300880]: [2026-01-17 00:20:02] INFO root virtnbdbackup - main [main]: Temporary scratch file target directory: [/var/tmp] Jan 17 00:20:02 serenity vm_backup.sh[300880]: [2026-01-17 00:20:02] INFO root job - start [main]: Starting backup job. Jan 17 00:20:02 serenity vm_backup.sh[300880]: [2026-01-17 00:20:02] INFO virt client - startBackup [main]: Setting supported flag to prevent vm shutdown during backup. Jan 17 00:20:02 serenity vm_backup.sh[300880]: [2026-01-17 00:20:02] WARNING fs fs - freeze [main]: argument unsupported: QEMU guest agent is not configured Jan 17 00:20:02 serenity vm_backup.sh[300880]: [2026-01-17 00:20:02] ERROR root job - start [main]: Failed to start backup: [unsupported flags (0x2) in function qemuBackupBegin] ```