Good. Assaid, the cardboard indicates oxidation. Likely entrained post boil at some point in your process. But, "maybe" a stretch could have some to do with the age of the flakes too. Just cause you had them a week doesn't mean they couldn' be years old. In fact, I don' buy adjuncts from my LHBS because they sit forever.
Maybe next time give it a go with Organic cereal flakes from a health food store. If the flavor re-occurs it may have something to do with your process and that the beer is so subtle it shows through where it otherwise wouldn't.
I assume this is a American lager? Thoise can be a b1tch to brew as they highlight everything that one can or could do without 100% precision.