Looks like you found the issue then.
Is the filer joined to AD at all?
If not, change the security style to UNIX.
If so, make sure your usermap entry syntax is correct.
https://library.netapp.com/ecmdocs/ECMP1196993/html/GUID-83730CE4-A3FC-46B5-918D-2790D205A22B.html
In your case:
domain\user == root
