Thank you Gergith for this thread!
It really helped me and I have my Kobo back from death.
There's one thing to mention, though.
The MiniTool application did not work for me. It somehow did not recognise the SD card after writing the image to it. I had to install another one which did the job (in my case: AOMEI Partition Assistant).
https://ibb.co/g1KhFk
Anyways, THANK YOU VERY MUCH!