Virtual Machine is unavailable and will ... - Veeam Software
https://www.veeam.com/kb129915.10.2011 · This error occurs when a virtual machine's unique inventory reference identification number (MoRef or Managed Object Reference) changes within the virtual infrastructure, or the virtual machine is deleted. The virtual machine is still in the Backup Job, and when the job starts, the job is still attempting to find the VM by its old reference ID.