I'd like to echo what @Killom is experiencing. I am using a group I manually added and am receiving this same error. It's causing an issue because I cannot edit any files within my shares, although I can delete/move/copy and create new ones. Any movement on this issue?
Groups with same names as users do not get into the group_mapping.tdb file and so aren't eligible. You can also try adding your user to the existing group "builtin_administrators".
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