The Jolla sbj-1 uses btrfs as device filesystem. Because the fs is so small, the whole mmcblk0 is only 16GB, it gets easily to a state where you have free space on the device but you cannot use it because btrfs needs to present allocated space in certain way.
The solution is to check the allocation and balance the btsfr slabs.
Unless you do this you will not be able to complete the upgrade and your device will not boot.