You are backing up several virtual machines with Acronis Agent for Hyper-V (installed on the Hyper-V host) or Acronis Agent for VMware (Windows) installed on a Windows machine connected to the ESX(i) host.
The backup plan is applied to a group of machines.
In a while, you receive an error message or an email notification about the error:
Error code: 58
Message: No backup items were resolved: ('500EC4AF-FFF4-A630-17E6-FBB30CFD4DC3','mms::vm::machine') GXT: The item provider has skipped the report of an item with key '('500EC4AF-FFF4-A630-17E6-FBB30CFD4DC3','mms::vm::machine')'.
This error means that a virtual machine was disconnected (deleted or migrated) from the original host, while it is still listed in the backup plan.
This is a known issue with agentless backup of virtual machines; the backup plan is not automatically revoked once the machine is no longer present in the environment.
The issue is fixed in Acronis Backup Cloud 6.1.
As workaround, revoke and then re-apply the backup plan to the host.