Comment 252 for bug 1268257

Dpkg-reconfigure does not seem to work.

From dmesg:
[ 46.423333] nvidia 0000:01:00.0: irq 85 for MSI/MSI-X
[ 46.935467] NVRM: Your system is not currently configured to drive a VGA console
[ 46.935469] NVRM: on the primary VGA device. The NVIDIA Linux graphics driver
[ 46.935470] NVRM: requires the use of a text-mode VGA console. Use of other console
[ 46.935471] NVRM: drivers including, but not limited to, vesafb, may result in
[ 46.935471] NVRM: corruption and stability problems, and is not supported.
[ 47.136998] Bridge firewalling registered
[ 47.296399] ip_tables: (C) 2000-2006 Netfilter Core Team
[ 47.313931] nf_conntrack version 0.5.0 (16384 buckets, 65536 max)
[ 47.330843] IPv6: ADDRCONF(NETDEV_UP): virbr0: link is not ready
[ 47.338417] init: plymouth-upstart-bridge main process ended, respawning
[ 47.340651] init: plymouth-upstart-bridge main process (2013) terminated with status 1

After this, trying to login on gdm (gnome-session and system-default-session), a black screen appears.
After logging on tty0, I got:
Failed to start unit user@1000.service

From there, I restarted gdm service and I was able to loggin.
However the problem persists after rebooting.

Dmesg after gdm restart:
[ 47.434555] systemd-logind[2143]: Failed to start unit user@116.service: Unknown unit: user@116.service
[ 47.434559] systemd-logind[2143]: Failed to start user service: Unknown unit: user@116.service
[ 47.436677] systemd-logind[2143]: New session c1 of user gdm.
[ 47.436688] systemd-logind[2143]: Linked /tmp/.X11-unix/X0 to /run/user/116/X11-display.
[ 51.028511] audit_printk_skb: 36 callbacks suppressed
[ 51.028513] audit: type=1400 audit(1428492782.685:39): apparmor="DENIED" operation="open" profile="/usr/lib/telepathy/mission-control-5" name="/etc/dconf/profile/gdm" pid=2589 comm="mission-control" requested_mask="r" denied_mask="r" fsuid=116 ouid=0
[ 97.512439] traps: gnome-shell[2886] trap int3 ip:7fa8dccf8d30 sp:7fff31d7e670 error:0
[ 98.281156] systemd-logind[2143]: Failed to start unit user@1000.service: Unknown unit: user@1000.service
[ 98.281186] systemd-logind[2143]: Failed to start user service: Unknown unit: user@1000.service
[ 98.283358] systemd-logind[2143]: New session c2 of user ****.
[ 113.017032] systemd-logind[2143]: New session c3 of user gdm.
[ 113.923759] traps: gnome-shell[3502] trap int3 ip:7f07ab6e7d30 sp:7fff9e673d80 error:0
[ 115.792429] audit: type=1400 audit(1428492847.393:40): apparmor="DENIED" operation="open" profile="/usr/lib/telepathy/mission-control-5" name="/etc/dconf/profile/gdm" pid=4205 comm="mission-control" requested_mask="r" denied_mask="r" fsuid=116 ouid=0
[ 120.230229] systemd-logind[2143]: New session c4 of user ****.