Comment 4 for bug 1066144

I don't believe this is an Upstart issue: The current version of Upstart does have code to call dbus_server_disconnect(), but this code is never actually run (since Upstart never "shuts down" itself). Additionally, if it were an Upstart issue, according to that log, the assertion would have failed on boot, which would have caused Upstart to crash and a kernel panic to ensue.

So, there must be some other application that is using D-Bus incorrectly. I'd suggest looking at the rdepends for libdbus-1-3. I wonder which set of postfix packages you have installed too? Also, you could conceivably disable each SysV init script and Upstart job in turn for the set of packages that trigger the problem to help hone in on the issue (I've installed "postfix" but cannot force the issue).