I just made the upgrade with the latest version (September the 3rd I think). I had to update TWRP first, I also flashed the Pico Open gapps package and everything gone fine. After first boot I could read/write my SD card but after a reboot, it becomes “Corrupted”. Lineage OS offers to format SD card to “repair it” but it remains “corrupted”. A mentioned by Christian_Michel above, I can read my SD card on a computer, so after a backup, I’ll try to format it to ext4 too.
P.S.: should have post it in “lineage-15.1-20180903-nightly-FP2-signed.zip feedback thread”