For the record, I've this exact problem on Asus Zenbook UX305LA. As stated in [1] the issue seems to come from gnome-settings-daemon trying to set backlight as root, which fails.
Log flood can be stopped with acpi_als blacklist adding blacklist acpi_als in /etc/modprobe.d/blacklist.conf Cheers, Jean Baptiste [1]: https://bugs.launchpad.net/ubuntu/+source/gnome-settings-daemon/+bug/1584457
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