After a tough battle (flashing a stock ROM to factory settings from the stock ROM), multiple factory resets under Android Pie, and re-flashing TWRP, flashing /e/, a PIN is requested on the first boot (?).
I hadn’t assigned one in the stock Android?
So I went back into recovery, which was then the one from /e/, another factory reset (the fourth in total!).
The installation that was then carried out (2.8) then presented no more errors when assigning the PIN and setting up the fingerprints!