i managed to downgrade to version 1.3 with a backup i had made with droid - tools. I used flash-tools, formatted the partition (without bootloader), then flashed the backup-recovery. everything went fine, i have Fairphone 1.3 again and the flickering is gone. it definitely has to do with the update!
but remember to write down the imei1 and emei2, because i had to restore them after the flashing, (its easy done with droid-tools). i think its important to know, that this flashing-back to fp1-1.3 was necessary on my daughters fairphone because of the screen-flickering, it was almost unreadable. with my own fp1 i had no problems at all after the update to 1.6.