Took me a while to find a solution and this thread was one of those that kept coming up on top of Google, so for any one wondering why...
SMB1.0 is deprecated in Windows 2012R2, by default a minimum of SMB2.0 is required.
The fix is from technet (search for cant-access-unc-share-on-windows-server-2012-r2), which reenabled SMB 1.0
Original Value: SamSS Srv2
Change to: SamSS Srv
Then reboot / restart the service.
This fixed Samba 4.1.5 and 3.6.18