I managed to build a new image too. There were some files missing from originial, if I found all that is. I added them from my regular linux machine as there were all public files like /zoneinfo folder and xmls in /application.

But:
When I write the new image to mmcblk0p28 everything seems fine, btrfs check gives no errors.
After reboot, I get the "trying to recover" followed by "seek service" I mentioned above.
When doing another btrfs check, file system is now broken again with lots of csum erros and more.

What am I missing?

btw: asking on #jollamobile or zendesk seems to not help. Please help somebody!!