A user reported that when they upgraded from 14.04 (upstart) to 16.04 (systemd), the agent would no longer start.
I advised them to try to do a sudo apt-get install -f appcanary to reset it to use systemd.
I will update this issue to report if that fix worked. In the future we should be able to specify to apt somehow that when you do a dist upgrade, install the version of appcanary for your distro?