Hello @ozehnder
looking at the factory test source code, error 0x21 appears when the AXP192 temperature reading either is below 5 or above 90 - which is used as an indicator that communication with AXP192 cannot be established.
I probably would try to run the AXP192 example to see if the AXP192 really refuses to communicate and if yes contact the store for a replacement.
Thanks
Felix