Hi marmistrz,
yes 'ubiboot' is a big thing with some challenges.
As second kernel needs to be loaded from first kernel (with his root) from any location on any partition you need to specify the full partition path.
You gave full path inside your new rootfs only (/boot/kernel.name)
but you need to specify the partition also. And as juiceme (HUGE thanks to him, cannot stop to do so ) created it like OS names instead partition numbers, you will have to specify
/boot/Debian/boot/kernel.name