If the problem is disambiguating AttrKeyList and AttrAssociation, how about simplifying the grammar by eliminating the AttrKeyList rule and augmenting key_value to include just a plain string:

key_value: string '=' string | string
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This will allow things like:

        UserNames = { vcs = X1Nh6WIWs6ATQ, foo }
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which wasn't legal in your original grammar. However, you can always check for a proper AttrKeyList or AttrAssociation after you've parsed the input.