Ah no, that was a misunderstanding.
It's https://sell.tschunk.shop/bar.json which has no authentication configured.
The API endpoint to generate the offer
https://pay.tschunk.shop/[https://pay.tschunk.shop/instances/bar/private/orders]instances[https://pay.tschunk.shop/instances/bar/private/orders]/bar/private/orders[https://pay.tschunk.shop/instances/bar/private/orders]
has proper token auth.
Can you check whether the json above specifies the api key correctly?
Also, where did you get that error message? I couldn't find any "error log"
button in the POS app.