In defense of everyone suffering awkward stuff with his Jolla, i have been there... I feel your pain!

There is one major flaw on Sailfish most users have not realized, because they should not have to!

DEM CALL IT BTRFS o.=

Sorry for the loud words. But every single problem i experienced on three Jolla Phones in my family and two with friends, where rooted to btrfs problems.

The observation for me is, humble little i (always 5 gig space available) never had one single problem ever since jan/2014.
My loving wife shooting video like there is 128gb space and my daughter downloading gigs of android games manage to destroy their devices (in their eyes) every second month.

I experienced failed system updates due to btrfs,
hanging and lagging ui due to btrfs,
spontaneos reboots due to btrfs,
and just for the sake of not writing due to btrfs a fourth time i will stop here.
The main problem is that the disk is not actually full, showing up to 4gigs available but somehow cluttered from e.g. moving large files.

So, please before giving up, try the btrfs balance.
go in your terminal
if the "used" allocation is higher than 13153337344 bytes
Even a btrfs balance and reinitializing the last system-update did not help in one occasion on my daughters phone, so i did a backup, factory-reset, reinstall apps, restore backup and all is fine still then.

My conclusion, try in order 1. balance, 2. reinitiate last system-update. 3. factory-reset.
If problems persist, it is a HW defect and still worth a try on warranty.