Not sure if this is a frab-issue, but
https://fahrplan.events.ccc.de/congress/2018/Fahrplan/qrcode.html is showing a QR-code for the URL https://fahrplan.events.ccc.de/congress/2018/Fahrplan/, which Giggity can't use -- it needs a trailing schedule.xml, i.e. https://fahrplan.events.ccc.de/congress/2018/Fahrplan/schedule.xml