Advice, please: Fairphone 2 won't boot and TWRP factory reset doesn't help

[AnotherElk]

FP2: Fairphone Open 18.10.0 (Android 7.1.2)

The first reboot after installing TWRP needs to be into TWRP. If the first boot after installing is into the normal system, the old TWRP version will be restored.