Environment
Open Enterprise Server 2018 Support Pack 3 (OES2018 SP3)
Open Enterprise Server 2018 Support Pack 2 (OES2018 SP2)
Open Enterprise Server 2018 Support Pack 1 (OES2018 SP1)
Open Enterprise Server 2018 (OES2018)

Situation
Unknown users listed in trustee assignments

Cause
The user used to exist and has been deleted from eDirectory but the rights assignments remain for NSS.

- In iManager > Roles and Tasks > Files and Folders > Properties > select a volume > select Rights.
- On this screen there are listed multiple "unknown users" in the list of Trustees.
- Some of the unknown users may have different rights than other unknown users.
- This indicates that these unknown user links used to refer to actual user objects in eDirectory.
- If you click on an unknown user you will get -601.  The -601 error tells you that the user that this link used to point at is no longer an object in eDirectory

Find resolution on support portal here