Thank you very much Renate.
That was indeed the problem, and what took me time to realize yesterday is that I could sideload a new firmware from recovery to start from scratch.
The system is no more bricked and I even allowed myself to start over: it is now successfully rooted!
However, I no longer have WiFi: impossible to activate it! It's not that it refuses to connect, it won't even turn on and scan. I tried from the console without any more success. I tried to reset the device several time with and without the patch. I finally wrote to Boox.
Code:
$ cmd wifi status
Wifi is disabled
Wifi scanning is only available when wifi is enabled
BOOX:/ $ cmd wifi set-wifi-enabled enabled
BOOX:/ $ cmd wifi status
Wifi is disabled
Wifi scanning is only available when wifi is enabled
Thank you again, and even though I'm struggling, I'm learning a lot of things.