VADP backups receive File....(Access is Denied)

We are running Networker 7.6 SP3 on Windows 2008 SP2 servers (Storage Node and Networker Servers), and using it to backup VMs (again Win 2008 SP2), from ESXi/vsphere 4. Out of around 100 VMs, they all back up fine using VADP apart from 6 of them, which have multiple warnings in the format : File C:\users\Jblogs\... cannot be opened so was not backed up (Access is Denied).

The VMs are successfully having a snapshot taken and being mounted on the VADP proxy node. We are using SAN transport mode.

Each if these files that cannot be backed up is one of the "Windows Special Folders" e.g. Nethood or Printhood. The Networker logs, (Daemon,messages and \tmp\sg) do not actually contain any errors, just the "messages" log has plenty of in, yet the job gets flagged as failed despite the group success threshold setting being set to warning, which should mean it succeeds, shouldn't it?

So my questions are:

Why, if these are only warnings does the job get flagged as failed, when we have set the success threshold to "warning"?

Why does this only happen on some servers and not more widespread as all the servers have profiles on them in C:\users.containing these Special folders.

Has anyone seen this and got a resolution?