to me it is just a stale taste.
If it is quick oxidation this is what I get as well. The longer it develops the more "sherry wine" like it gets. The wet cardboard is a decent analogy as well.
And I'm glad Gordon also mentioned the weird caramel taste that oxidized beers have. That's something I perceive, too, that you don't see get mentioned much.
Right. I notice a shrinkage of flavor level or less intense (brightness or freshness) flavors. Boxy or cardboard and sherry are classic descriptors which to me are most accurate. I've also experienced some slightly metallic flavors in older beers.
I think Gordon hit the high notes of this condition. The best way to recognize this is to do a blind tasting of like examples a few years apart, of course easier said than done.