Colleagues, could you please advise what a problem could be in XML file in case of:

virsh backup-begin lubuntu2 ./backup.xml

where backup.xml -

<domainbackup>
    <disk name='/var/lib/libvirt/images/lubuntu2.qcow2' type='file'>
    <target file='$PWD/scratch1.img'/>
    <driver type='raw'/>
    </disk>
    <disk name='/var/lib/libvirt/images/lubuntu2-1.qcow2' type='file'>
      <target file='$PWD/scratch2.img'/>
      <driver type='raw'/>
    </disk>
</domainbackup>

and domain config is-

<domain type="kvm">
  <name>lubuntu2</name>
  <uuid>c466086e-4570-4fb5-8cb6-bfe104791381</uuid>
  <memory unit="KiB">1048576</memory>
  <currentMemory unit="KiB">1048576</currentMemory>
  <vcpu placement="static">1</vcpu>
  <os>
    <type arch="x86_64" machine="pc-i440fx-focal">hvm</type>
    <boot dev="hd"/>
  </os>
  <features>
    <acpi/>
    <apic/>
    <vmport state="off"/>
  </features>
  <cpu mode="host-model" check="partial"/>
  <clock offset="utc">
    <timer name="rtc" tickpolicy="catchup"/>
    <timer name="pit" tickpolicy="delay"/>
    <timer name="hpet" present="no"/>
  </clock>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <pm>
    <suspend-to-mem enabled="no"/>
    <suspend-to-disk enabled="no"/>
  </pm>
  <devices>
    <emulator>/usr/bin/qemu-system-x86_64</emulator>
    <disk type="file" device="disk">
      <driver name="qemu" type="qcow2"/>
      <source file="/var/lib/libvirt/images/lubuntu2.qcow2"/>
      <target dev="hda" bus="ide"/>
      <address type="drive" controller="0" bus="0" target="0" unit="0"/>
    </disk>
    <disk type="file" device="cdrom">
      <driver name="qemu" type="raw"/>
      <target dev="hdb" bus="ide"/>
      <readonly/>
      <address type="drive" controller="0" bus="0" target="0" unit="1"/>
    </disk>
    <disk type="file" device="disk">
      <driver name="qemu" type="qcow2"/>
      <source file="/var/lib/libvirt/images/lubuntu2-1.qcow2"/>
      <target dev="hdc" bus="ide"/>
      <address type="drive" controller="0" bus="1" target="0" unit="0"/>
    </disk>
    <controller type="usb" index="0" model="ich9-ehci1">
      <address type="pci" domain="0x0000" bus="0x00" slot="0x05" function="0x7"/>
    </controller>
    <controller type="usb" index="0" model="ich9-uhci1">
      <master startport="0"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x05" function="0x0" multifunction="on"/>
    </controller>
    <controller type="usb" index="0" model="ich9-uhci2">
      <master startport="2"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x05" function="0x1"/>
    </controller>
    <controller type="usb" index="0" model="ich9-uhci3">
      <master startport="4"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x05" function="0x2"/>
    </controller>
    <controller type="pci" index="0" model="pci-root"/>
    <controller type="ide" index="0">
      <address type="pci" domain="0x0000" bus="0x00" slot="0x01" function="0x1"/>
    </controller>
    <controller type="virtio-serial" index="0">
      <address type="pci" domain="0x0000" bus="0x00" slot="0x06" function="0x0"/>
    </controller>
    <interface type="network">
      <mac address="52:54:00:90:d7:29"/>
      <source network="default"/>
      <model type="e1000"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x03" function="0x0"/>
    </interface>
    <serial type="pty">
      <target type="isa-serial" port="0">
        <model name="isa-serial"/>
      </target>
    </serial>
    <console type="pty">
      <target type="serial" port="0"/>
    </console>
    <channel type="spicevmc">
      <target type="virtio" name="com.redhat.spice.0"/>
      <address type="virtio-serial" controller="0" bus="0" port="1"/>
    </channel>
    <input type="tablet" bus="usb">
      <address type="usb" bus="0" port="1"/>
    </input>
    <input type="mouse" bus="ps2"/>
    <input type="keyboard" bus="ps2"/>
    <graphics type="spice" autoport="yes">
      <listen type="address"/>
      <image compression="off"/>
    </graphics>
    <sound model="ich6">
      <address type="pci" domain="0x0000" bus="0x00" slot="0x04" function="0x0"/>
    </sound>
    <video>
      <model type="qxl" ram="65536" vram="65536" vgamem="16384" heads="1" primary="yes"/>
      <address type="pci" domain="0x0000" bus="0x00" slot="0x02" function="0x0"/>
    </video>
    <redirdev bus="usb" type="spicevmc">
      <address type="usb" bus="0" port="2"/>
    </redirdev>
    <redirdev bus="usb" type="spicevmc">
      <address type="usb" bus="0" port="3"/>
    </redirdev>
    <memballoon model="virtio">
      <address type="pci" domain="0x0000" bus="0x00" slot="0x07" function="0x0"/>
    </memballoon>
  </devices>
</domain>