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.

As far as I understand it, Sailfish runs btrfs balance on the background every night (maybe only if on the charger? can anyone confirm?). You do not need to do anything special, but leave the device settle between updates as chemist suggested. At least overnight.

Yes and No.

That was not the behaviour in the first releases, cannot remember when it started doing that but it was maybe an year after the first release.

Also that is not valid any longer, in some release (2.0.something) the feature was removed.

The reason I suggest doing it manually between each update is exactly the reason you can be sure your FS is clean before attempting the next update.

Ok but how to do it?
can you give me code please btw each update.

Check the FS; "devel-su btrfs-balancer allocation"
Balance the FS; "devel-su btrfs-balancer balance"

I had no problems (months ago) to update two J1s from 1.0.0.5 to newest (2.xxx).
And absolutely no probs.
Balancing should not be the issue as the amount of download data is not really that much (8*500MB~4GB) should not cause the btrfs balancing "bug".