Comment 44 for bug 1359439

I might come after the war has ended but after a lot of investigation about my problem, I found something which might be interesting for you.

First of all:
Ubuntu gnome 14.10 (clean install)

What was my problem:
Since today (1st of March : date is important here) GDM did not show up (black screen).
From my terminal, when loging in, I had the following 2 lines
[ 7.287663] systemd-logind[1057]: Failed to start unit user@1000.service: Unknown unit: user@1000.service
[ 7.287677] systemd-logind[1057]: Failed to start user service: Unknown unit: user@1000.service
But I could do what I wanted and start looking for a workaround (like using lightdm)

How I fixed my initial problem: I changed the language of the login screen for French (instead of German).
Why: Using lightdm, I saw that the month of the today date was '(null)'. In German, March is März ... So I supposed any encoding compatibility problem occured, probably better managed with lightdm (null) than with gdm (hangs).
And: It also removed the 2 error lines of systemd-logind

Voilà,
I hope it will help anyone