Enable signing on the third-party file server.
NetApp CLI:
options cifs.smb2.signing.required on Disable "Secure Negotiate" on the client.
PowerShell in Windows8 oder Windows Server 2012:
Set-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters" RequireSecureNegotiate -Value 0 -Force