Continuing the discussion from 
FP2 startet nicht mehr:
You said it could take some time. The installation via TWRP went smoothly, but after rebooting the phone it displays for approx. 1 1/2h “Android wird aktualisiert - Einstellungen wird vorbereitet” (should called “Android is being updated - settings are prepared” or something like that). Is this the long time you meant? I’m afraid of getting off the battery, but don’t know what to do else.