Just as a lessons learned for all who may encounter similar strange problems:
It is BTRFS failure,... due to probably too much data.
But we may never know as it was even too much to mount on my Ubuntu laptop,...

Solution level1: https://together.jolla.com/question/...in-write-mode/

I failed at level 1 as I couldnt mount nor repair even via my laptop.

Solution level2: You have a clean OS immage available (maybe your own saved as backup) what you can replace the corrupted bits.

Level 2 is a lost cause for me as well as I have no such immage available nor an other J1 to copy it from. Coderus used to provide one some time ago but was removed for some reasons. (Qualcomm related copyright???)

So if anyone would be so kind to give me a clean [ /dev/mmcblk0p28 ] straght after device reset (so no private info is inside) I would happily revive my J1.

Until that I have to live without her,...