Unifi u6+ failing to upgrade

I have quite a few sites where some Unifi U6+ Access Points fail to upgrade with a generic update failed message. Marquis WiFi Vergaderruimte 4 update failed. I've tried everything, from ssh 'ing, factory resetting with set-default , to manually upgrading with upgrade , etc. Nothing worked. I thought I had a bunch of bad APs (and many had their warranty expire by a few weeks or months). As a last…