Comment 3 for bug 1161329

So you apparently ran into two problems:
 - You're running a kernel < 3.5 so you're missing the subreaper support in the kernel. That's not a big problem as upstart handles that gracefully but you won't be able to track processes for jobs doing double-fork.
 - You are apparently using an old version of libnih. You really should backport the one we currently have in Ubuntu as it includes dbus fixes which solve the crash you're getting. Note that dropping --session won't make you immune from that specific crash, so you really should get a newer libnih on the system and rebuild upstart with it.