Thanks for your reply.
Well, I’ve tried to use again the original boot.img but the update failed anyway.
I do understand your reply, the thing is, at time being, I can not check if it’s applies to the Fairphone, since even on Android 5 the partial update did not work on the rooted system, we had to use to “full update file” to update the system.
With the last update from 17.04.3 to 17.04.8, this file is not available so I can not try the way I’ve done for months with FP 2 GMS verison 1.XX.YY.
I hope I’m clear…
Maybe a solution is to use TWRP to apply the minor update, maybe it will accept to update even on a rooted phone.