If disabling autobrightness fixed it per comment #6 there is some chance its related to a defective sensor. Did you re-enable autobrightness and is that how the issue recurred?
FWIW I do not see anything unusual in the last syslog -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to powerd in Ubuntu. https://bugs.launchpad.net/bugs/1582337 Title: sudden black screen Status in Canonical System Image: Confirmed Status in powerd package in Ubuntu: Confirmed Bug description: When working on MX4 the screen (backlight?) suddenly becomes black. For example whilst playing balls, browsing, aso, the backlight seems to turn off so that you can hardly see anything. A reboot does not reset the backlight. You need to shut down the phone and start it again. In a dark room you are able to see the dialogues. In daylight it's impossible. Issue occured first times a week ago, sometimes it occures four times a day, some days just once. Phone MX4, stable, 10.1 (2016-0408-211605) To manage notifications about this bug go to: https://bugs.launchpad.net/canonical-devices-system-image/+bug/1582337/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : touch-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp
