Have you tried keeping the screen “active” permanently for the duration of the installaion?
How-to: settings > developer options and tick “keep screen on” and then start the update with your phone plugged in" (see ✏ How to update your Fairphone 1(U))
Regarding the Fairphone Updater: it crashes on starting in less a second. And as for the manual install the screen does not fade. So, I didn’t try to check this option, since the screen does not turn off.
Make sure, you are using the correct version of Fairphone OS 1.8.7. Note that there is one for the old partition layout and one for the new partition layout. Then download the file again. It might well be with a 200MB file that it got corrupted during download.
After downloading it and put it on a place in the Fairphone. You have to shut down the Phone.
When you hold at the same time, the Volume Up-Button and the ON-Button. Wait 10sec.
There will a Screen with android robot. If not put short one more time on the On-Button.
The Scroll with the Volume down- button, Start with the up-Button.
Apply update from the “Download” folder on the Fairphone’s internal memory. Do this by choosing “apply update from sdcard”, and navigate to the “Download” folder. Select the .zip file (make sure you select the .zip file starting with “FP…” not “._FP…”) in there and press the Volume Up button.
So i hope u will be faster than me.
Maby you could write a Helpfile on at other topic.