vmware backup failure error code 156

Also check VMs for pending consolidation tasks.  Sometimes the vSphere 'manage snapshots' view doesn't list snapshots, but they do still exist - if you look in the VMFS/NFS datastore folders... and the only way really to see these is either browse the datastore, or change your vSpehere management GUI 'view' to also include the column which shows pending consolidation.

If you do see pending consolidation, but no 'snapshots' in the GUI, but you do see additional snapshot related VMDK files in the datastore folders... then... do NOT be tempted to delete the snapshot files unless you are absolutely 100% certain that such 'delete' action is approrpriate... BECAUSE...

...vSphere/ESXi 'snapshot files' are actually 'pending write logs' - and not at all like discardable VSS delta change logs - i.e. you need to remember that vSphere snaphots contain 'writes' which have not yet been 'consolidated/merged' into the main VMDK files.

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And so, sometimes the pending consolidation stops NetBackup from being able to take its (additional?)  snapshot.