Why does a backup take so much time?

Hello! On ARM Recovery Services i have an error: "Backup Failed, FailureDetails:ErrorCode: DiffCopyFailedToTransferData". What does it means and how to troubleshoot it?

We provided: aka.ms/d1137882

The reason is "Another operation is currently in progress on this item. Please wait until the previous operation is completed, and then retry". I thought this backups is incremental and should be fast. Why do they takes so much times?                                       

Thanks,

@AzureSupport