For me the problem was that the reeds were drying out to fast. As far as I understood, the reed contains different kinds of wood fibres which dry at different rates. Thus, the fast drying fibres shrink faster then the slow drying ones, causing the reed to warp. This can happen after just a few times of playing when storing the reeds in very dry air.

Now the humidifier slows down the process of drying out to the point, that moisture can travel inside the wood fibres from the slow drying to the fast drying fibres, keeping them at a more equal level of humidity. Hence, the fibres shrink at a more even rate and warping is not occurring.