The QR code links in my book were broken for some time, but they work again!
Discussion
In case you're wondering... I was hardcoding the php version in nginx config and forgot to update it when Ubuntu bumped it from 8.2 to 8.3.
Finally learned this one great trick to fix that once and for all: /run/php/php-fpm.sock
https://serverfault.com/questions/1155870/php-sock-linked-to-update-alternatives-choice