The infamous cardboard taste

I got it on my third batch. I understand it is due to oxidation but I still new to this. I did not add cooled water to bring it up to 5 gal when I transferred from wort to primary. In the secondary fermenter it was about 2/3 full (5gal glass carboy). Could this have caused the problem? The beer is a nut brown ale that I added honey and an extra lb of malt extract to get hight alc%. Does this also cause a cardboard taste?

Did you adjust your recipe to compensate that you weren't brewing 5 gallons? Making a 5 gallon batch with only 2/3 the water seems like it would result in a pretty malty beer.

What temp did you pitch yeast at. Did you aerate your wort. and at what temp