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BugZero found this defect 2020 days ago.
When backing up a VM within an environment running vSphere 5.5 or vSphere 6.0 the job fails with:
This failure is related to a known issue affecting VMware Virtual Disk Development Kit (VDDK) 6.0. VDDK 6.0 is used by Veeam Backup & Replication to access VMs running in Vsphere 5.5 and 6.0.
Disable IPv6 on all network interfaces of the Veeam Backup Proxies.