Update: I installed dropbear via Diags and had a look around at the system but wasn't able to find out, what was missing. I didn't change anything 'massive' in the past for sure (was only interested in SSH+VNC to use it as a display), but I guess I installed some kind of 'Updates Block' to avoid loosing the jailbreak.
In the end I needed some hours to make Kubrick work (see my post there:
https://www.mobileread.com/forums/sh...postcount=1202). But I was able to reinstall the kindle and install all updates + jailbreak it again.
I guess this device is simple turning a bit old, and many people in this forum (which did great work on all kind of kindle software) already left or don't support K4NT anymore (sadly found many dead links on my journey to repair this device).
Still, thank you all!