After some investigation, it turned out that the Google Adsense code started to change the styles of the page it is on.

It seems it figures out which parent element or grand-parent or grand-grand-parent... element is limiting its height. And then adds "height: auto !important" to that element.

If it is limited by max-height, it adds "max-height: auto !important" to the limiting element.

I had Googles dynamic ad sizes in a div with a size that fits my layout. Since they adapt to the space they get, everything was fine. But now they explode their parent element to a size that breaks my site.

I find this behavior pretty shocking.