Yes, I have an internal PD bug open. This is an interop issue between the filer and ARX. Working with Engineer's in India with little response. No fault of there own considering the time difference and the fact that XP, 7 clients do work.
Most use pcuser but we have had root set in some installs...as long as there is a mapping that is what is needed. You can modify the default to pcuser and make sure the unixuser exists for pcuser but doesn't sound like the cause.
ONTAP just requires a mapping for default unix_user for cifs.. odd but just used to it for years now. Or unless you had explicit mappings for each user...but when native cifs it isn't using the 65534 uid for access.