Would anybody with data center experience be able to hazard a guess on what type of commercial resolution Hetzner would have reached with the Motherboard supplier here? Would we assume all mobos replaced free of charge plus compensation?
When you buy name-brand servers you'll definitely get any faulty hardware replaced. Compensation would only happen if you negotiated for that and you'd have to pay extra. You're probably better off buying some kind of business interruption insurance instead of trying to get vendors to pay you for downtime (even if it is their fault).
Hetzner is not a normal customer though. As part of their extreme cost optimization they probably buy the cheapest components available and they might even negotiate lower prices in exchange for no warranty. In that case they would have to buy replacement motherboards.
I think they probably got a batch of these really cheap in the first place, because those servers were offered without the setup fee initially. It was during the soccer World Cup in Germany.
