What is this error in windows server backup?

Question

A scheduled backup is being made by Windows Backup to a dedicated external storage.

However, the backup is made with some errors in some files and I do not have full knowledge of why this is happening.

The Hyper-V VSS Writer Status is stable.

How can I solve this problem?

Errors:

Writer Failures
Writer Id: {66841CD4-6DED-4F4B-8F17-FD23F8DDC3DE}
Instance Id: {F6830DA0-05EE-42B7-AF71-028D2F70D013}
Writer Name: Microsoft Hyper-V VSS Writer
Writer State: 5
Failure Result: 80042336
Application Result: 80004005
Application Message: (null)
   Component: FDCE592D-9C45-475B-BA6B-BB6C9258B186
   Logical Path: (null)
   Component Result: 800423F4
   Component Message: Could not create backup checkpoint for virtual machine 'FW01': Unspecified error (0x80004005). (Virtual machine ID FDCE592D-9C45-475B-BA6B-BB6C9258B186)
       File Spec: D:\VMachines\FW01\Virtual Machines\\FDCE592D-9C45-475B-BA6B-BB6C9258B186.vmcx Recursive: 0
       File Spec: D:\VMachines\FW01\Virtual Machines\\FDCE592D-9C45-475B-BA6B-BB6C9258B186.VMRS Recursive: 0
       File Spec: D:\VMachines\FW01\Virtual Machines\\FDCE592D-9C45-475B-BA6B-BB6C9258B186.vmgs Recursive: 0
       File Spec: D:\VMachines\FW01\Virtual Hard Disks\\FW01.vhdx Recursive: 0
       File Spec: D:\VMachines\FW01\Snapshots\AE3794CF-1BB6-4EA9-B0E0-D3F473B55139.VMCX Recursive: 0
       File Spec: D:\VMachines\FW01\Snapshots\AE3794CF-1BB6-4EA9-B0E0-D3F473B55139.VMRS Recursive: 0
       File Spec: D:\VMachines\FW01\Snapshots\AE3794CF-1BB6-4EA9-B0E0-D3F473B55139.vmgs Recursive: 0
       File Spec: D:\VMachines\FW01\Virtual Hard Disks\FW01_AE3794CF-1BB6-4EA9-B0E0-D3F473B55139.avhdx Recursive: 0
*-----------------------------*


Application backup
Writer Id: {66841CD4-6DED-4F4B-8F17-FD23F8DDC3DE}
   Component: FDCE592D-9C45-475B-BA6B-BB6C9258B186
   Caption     : Online\FW01
   Logical Path: 
   Error           : 80780175
   Error Message   : Component was skipped from volume shadow copy.

   Detailed Error  : 800423F4
   Detailed Error Message : The writer experienced a non-transient error.  If the backup process is retried,
the error is likely to reoccur.

Status of Hyper-V VSS Writer:

Writer name: 'Microsoft Hyper-V VSS Writer'

Writer Id: {66841cd4-6ded-4f4b-8f17-fd23f8ddc3de}

Writer Instance Id: {f6830da0-05ee-42b7-af71-028d2f70d013}

State: [1] Stable

Last error: Unexpected error