It wasn't only HACK that got broken today. Around the same time as HACK stopped working properly, Refined HN[0] (browser extension) did the same for me on desktop Firefox. It mostly works, but there is no more "favorite" button (and other extra buttons) next to individual comments and probably other features I am missing (because I don't use them).
Not sure if it is an HN API change or not, but feels like it.