I just ran this in my lab - works fine without the -accessrights parameter
Also, you could run the following command to remove all invalid users from public folder client permissions
Get-PublicFolder <TopOfFolderPathToStart> -recurse | Get-PublicFolderClientPermission | Where-Object {$_.User -like "NT User:*"} | Remove-PublicFolderClientPermission -Confirm:$false
