The ESTALE error simply means that the server couldn't find any file for a given file handle.
The most common cause of ESTALE is for a file that is open on one system to be removed on a different system. If the remove comes from the same system, then the NFS client detects that the file is open and renames it to ".nfs####" instead of removing it. But a remove from a different system has no way to detect that the file is open, since NFS is stateless, so the remove succeeds and accesses to the open file suddenly start failing with ESTALE.
This is, of course, not NetApp-specific; other NFS servers work the same way.