Grapical login fails (independent from Display Manager)

[Michael]

Symptoms:
- I cannot login to the graphical session neither using lightdm not lxdm
- After typing password and hitting enter the login screen disappears and reappears after 4-5 seconds
- Login as guest does not work as well
- Worked before, I am not aware of having done anything special
- Login on console works
- Tried several hints, like those from http://askubuntu.com/questions/223501/ubuntu-gets-stuck-in-a-login-loop, http://askubuntu.com/questions/590561/ubuntu-14-04-login-loop-problem
 - not a permission problem with .Xauthority, .ICEauthority, /tmp
 - not a graphics driver issue
 - sudo pam-auth-update --force did not help

According to lightdm.log, authentication succeeds and it stops greeter and starts gnome-session, which seems to fail with:
CRITICAL: session_get_login1_session_id: assertion 'session != NULL' failed

in ~/.xsession-errors I get:
openConnection: connect: Datei oder Verzeichnis nicht gefunden
cannot connect to brltty at :0
upstart: gnome-session (GNOME)-main-Prozess (2158) wurde mit dem Status 1 beendet
upstart: logrotate-main-Prozess (2023) wurde von TERM-Signal beendet
upstart: bamfdaemon-main-Prozess (2080) wurde von TERM-Signal beendet
upstart: Vom benachrichtigten D-Bus-Bus getrennt

[Michael (0815fox)]

Moved this to gnome-session, where I intended to post it.

[Michael (0815fox)]

I did not find a solution that repairs the issue, so I installed Ubuntu 16.04 LST freshly.

This morning I used the machine before work to book a train ticket and print is. After that I shut down the machine. 15 Minutes later I remembered that I forgot to lool up something and booted the machine again and cannot login anymore.