@NiLuJe
Thank you very much for your quick response!
I installed the new Dec-02-23 JB Hotfix as an mrpi package.
Then restarted Kindle Scribe two times:
1) First time it only said **** SQUASH JB ****
2) Second time it didn't say anything on the loading screen
Doubled checked the key in /etc/uks/ , everything is great! The key has root:root ownership (Owner:Group).