@chaitanyaI: yeah, it works for
Kindle Paperwhite 1 (generation 5) 
Thank you so much! Now I can read with KoReader again
The only step missing to make it work after updating KUAL and the certificate is to
also update the Kindle software to latest available version for this model.
I had 5.4.4 (not sure, something with 5.4) and I got the 5.6.1.1 update from Amazon:
https://www.amazon.com/gp/help/custo...MQC26VQQMM8XSW