Hi Community,
I decided to return to Android 6 and followed the steps " Fairphone OS downloads for Fairphone 2"
for Android 6.0
The phone got very hot and discharged within 60 Minutes. Good to know that all the apps got “optimized” , but the phone now loops in the boot process.
I found topic [Infinite boot loop - FP2]
(Infinite boot loop - FP2)
but the discussion is closed in the meantime 
Selecting the system.img I got the error message that the file is damaged.
Regards,
Katrin