Windows 8 and Windows 2012 introduced a "Secure Negotiate" feature to SMB, and they're configured to require Secure Server Signing by all SMB servers. The fix is to either tell the SMB server to enable signing (assuming your server is capable of signing), or to tell your Windows 8 client(s) to disable Secure Negotiate.
See Microsoft's description of this problem, or this page dealing with the same issue on a NetApp filer.
QNAP may have an upgraded firmware for your NAS which supports SMB 2.24 or newer, and the SMB signing necessary for native communication with newer Windows clients. They at least do for the Turbo NAS. So if you have a Turbo NAS, you could take this route.
Disabling Secure Negotiate on a Windows 8 client is easy, though. Start PowerShell, and enter the following command:
Set-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters" RequireSecureNegotiate -Value 0 -Force
Note that this can also be easily changed later with -Value 1
, if an upgrade to the NAS firmware comes later on.