I have a combination of errors reported elsewhere such as this logout problemlogout problem and this boot problemboot problem. Only that in my case it happens twice upon boot. First time after thermal daemon has started, the second just before or at the logon screen. It is not the thermald service though, as stopping it does not change the logout behaviour. Moreover, the computer goes to sleep every time I switch to a terminal, i.e. press Ctrl+Alt+F1. Never happened on 14.04., witnessed something similar on a test installation of arch, though.
Hardware is quite old. It is an Cebop Hel500 from 2005. That is based on a Fujitsu Siemens Amilo, so basically the hardware is all Intel.
I already tried to change the display manager (to slim), and I removed light-lock...in vain.