Comment 16 for bug 1510570

found a workaround to this:

if you happened to upgrade from vivid(15.04) or you know how to get vivid kernel install properly,
you can boot the system with 3.19 kernel and pairing up the mouse using bluetoothctl manually
then reboot with 4.2, the mouse will still paired and working.

$ bluetoothctl (entering interactive mode)
$ agent on
$ default-agent
$ pairable on
$ scan on
$ devices (to see the device mac that you want to pair)
$ pair <dev mac>
$ trust <dev mac>
$ connect <dev mac>

verified versions(stable while posted):
bluez 5.35-0ubuntu2
linux-image-3.19.0-43-generic 3.19.0-43.49
linux-image-4.2.0-23-generic 4.2.0-23.28