Just to close this topic on my side: I contacted OnlyKey support so they can honor their refund policy:
We only replace items if they are defective or damaged.
But they say that only applies to the first 30 days. They offer me a 25 USD discount for a new key, but they’re now really expensive. Now they cost 91,95 EUR, and I had paid 48,95 EUR for it.
I’ll not give more money to a product that stops working 14 months on normal use, and I prefer to pay something with a better warranty: here in Europe companies need to offer warranty for 3 years (36 months).
I don’t recommend this product anymore.