HyperV backups failing with error 156

There can be lots of reasons .. this link is pretty handy http://www.veritas.com/docs/000067928

bpfis logs on the media server are usually where you start but take  a look at the link

The NetBackup v7.6 EEB Guide shows, on page 14, that v7.6.1.2 includes a fix for:

ETrack: 3748081

Re: Hyper-V Backups on Windows 2012 R2 can exhaust system memory due to aggressive cache memory management. 

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FYI - the v7.6 EEB guide is here:

http://www.veritas.com/docs/000003552

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I've had a search through for other Hyper-V related issues, but any references to other known issues for Hyper-V backups are listed in the sections for other lower versions, and so, the only "known" (at the time) issue which may (or may not) be pertinent to you, because you are running v7.6.0.3, is the issue listed above which is documented as being fixed/resolved in v7.6.1.2.

HTH.