I just flashed a beaglebone black with this prebuilt ubuntu 14.04 image:
https://rcn-ee.com/rootfs/2015-12-11/flasher/BBB-eMMC-flasher-ubuntu-14.04.3-console-armhf-2015-12-11-2gb.img.xz
However the default ubuntu / temppwd username and password are not working. Do I have the wrong username and/or password?