I had the same issue when attempting to update my Pro 5 to OTA 14
I could not delete the folder contents, so kept getting device full
message.
I fixed it by reflashing right from custom recovery - /TWRP 3.0
m86.img (for Pro 5)
After installing TWRP, I booted into recovery and wiped cache and
partitions etc as if installing for the first time (e.g. as if over
the top of Android).
Then flashing the relevant Ubuntu Touch image - details I used are
here
https://www.smoose.nl/sites/default/files/bestanden/meizu_pro5_ubuntu.pdf
All now works fine.
It means though losing all current data, so you need to backup as
needed prior to wiping partitions