Thanks a lot to NoHaTaH on 4PDA for posting the passwords.bin file.
Xiaomi Mi Box S (MDZ-22-AB) [Android], [TV Box][Amlogic S905X]
4pda.to
Sucessfully unbricked and downgraded to Android 8 with 2.2.4 version using this file.
You can use this instruction on how to unbrick your device (you need the full recovery, with device disassembly and shorting the pin to shield/ground), but use the 2.2.4 version of Amlogin usb burning tool).
Guide to recover from a Brick the Xiaomi Mi Box and Xiaomi Mi Box S, unbrick the device from black screen or frozen.
androidpctv.com
You are about to download the Amlogic_USB_Burning_Tool_v2.2.0.zip file on your computer. The file comes as a zip package, and the file size is approximately 8.24 MB only. ADH Editorial Staff uploaded this file from Spain on 27/01/2021. The registered Editorial Staff uploaded this file...
androiddatahost.com
Update:
I've sucessfully unbricked it the second time. First time I got an error in chinese about u-boot, fix: just unplug the power, short again the pin with shield and connect the power back, don't touch anything in the app on PC.
Then I've copied the 8.1 update.zip to USB, updated it from recovery, disconnected the USB, copied the android 9 (3933) update.zip and updated again (it rebooted to install the new recovery and I started the update to 3933 again). The box is working properly, wifi is ok, the serial number and netflix keys are present.
Just don't forget to untick the "erase bootloader" and "overwrite keys" in the USB Burning Tool.