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