I’ve just found the solution for this that’s been bodering me for the last month or so. I filled up a bug at launchpad that contains all the info: https://bugs.launchpad.net/ubuntu/+source/lxdm/+bug/912772

After digging around I found that the upstart job /etc/init/lxdm.conf ran and stopped inmmediatly. A couple of weeks later (BTW, upstart debug log sucks!), I found that in this file there’s a condition that checks the content of /etc/X11/default-display-manager (line 28) and expects to found “/usr/sbin/lxdm”. In my case, the content of the file was “lxdm” therefore the condition was not satisfied and lxdm failed to start (actually it was being forced to stop).

After changing the content of /etc/X11/default-display-manager to “/usr/sbin/lxdm”, lxdm (LXDE) started to work at boot time showing login screen as expected.