Today I did a d-u on one of my own computers, I sad no to update the lightdm.conf but that didn't trigger the problem I had yesterday. The computer where the problem occurred was an older install (firestarter) than this computer 13.2.1 December) and the lightdm.conf was probably the original from the installation, never updated, I don't have access to that computer at home so I can't provide the files.