It can also happen (referring to the resources piece) if the request for a snapshot creation/removal takes too long to complete and the application time out period expires. This tends to be caused by limited resources on the vCenter server (specifically RAM) however can be caused by the vCenter and/or Rapid Recovery core rebooting during the course of an agent-less backup.
vmdks mounted to other VMs or perhaps other snapshot anomalies are afoot however this is the most common culprit with an agent-less backups solution active in the environment.
(if you do run across this error the Consolidate option from the snapshot menu in VMware will not fix this) Storage Performance and Utilization Management.Information Archiving & Storage Management.Hybrid Active Directory Security and Governance.
Starling Identity Analytics & Risk Intelligence.One Identity Safeguard for Privileged Passwords.