mjhjmtu

I think I messed up my Ubuntu 17.04 a little. In order to describe my problem I can only provide the symptoms + a suspicious log-file.

My quenstion on this is:
1.: Can I repair it by doing something with lightdm? (cf. below)?

Symptoms: When I start the pc, it never reaches the log-in-mask. Instead it seems to be trying to launch the graphical surface over and over again, but always fails to do so. I can hardly work on it in this state. I can switch to Ctrl+Alt+F2 and log in via terminal. Here I can do stuff, but roughly all 30seconds the monitor becomes black and shortly displays 'No signal'. After some seconds I can re-switch to there and start from were I left. Reboot does not help.

DISPLAY-variable is 1. $startx does not work.

System: Ubuntu 17.04, build pc, all parts were bought 2 months ago.

What I did: I installed pekwm and started it a few times, I also switched between the window-managers via $pekwm --replace& resp. $compiz --replace&. It worked fine.
I installed conky and wrote a .desktop-file to autostart it. This never worked. I deleted said .desktop-file, after the problem occurred.
Right before the problem occured, I do not recall having done anything else than firefox, LaTeX and mathematica.

It MIGHT be, that I messed up lighdm, without realizing. Here are last few lines of /var/log/lightdm/lightdm.log:

[+29.10s] DEBUG: Exiting with return value 1
[+0.00s] DEBUG: Logging to /var/log/lightdm/lightdm.log
[+0.00s] DEBUG: Starting Light Display Manager 1.22.0, UID=0 PID=5482
[+0.00s] DEBUG: Loading configuration dirs from /usr/share/lightdm/lightdm.conf.d
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/45-unity8.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-disable-guest.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-disable-log-backup.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-greeter-wrapper.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-guest-wrapper.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-unity-greeter.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-xserver-command.conf
[+0.00s] DEBUG: Loading configuration dirs from /usr/local/share/lightdm/lightdm.conf.d
[+0.00s] DEBUG: Loading configuration dirs from /etc/xdg/lightdm/lightdm.conf.d
[+0.00s] DEBUG: Loading configuration from /etc/lightdm/lightdm.conf
[+0.00s] DEBUG: Registered seat module local
[+0.00s] DEBUG: Registered seat module xremote
[+0.00s] DEBUG: Registered seat module unity
[+0.00s] DEBUG: Using D-Bus name org.freedesktop.DisplayManager
[+0.01s] DEBUG: Monitoring logind for seats
[+0.01s] DEBUG: New seat added from logind: seat0
[+0.01s] DEBUG: Seat seat0: Loading properties from config section Seat:*
[+0.01s] DEBUG: Seat seat0: Starting
[+0.01s] DEBUG: Seat seat0: Creating greeter session
[+0.01s] DEBUG: Seat seat0: Creating display server of type x
[+0.02s] DEBUG: Using VT 7
[+0.02s] DEBUG: Seat seat0: Starting local X display on VT 7
[+0.02s] DEBUG: XServer 1: Logging to /var/log/lightdm/x-1.log
[+0.02s] DEBUG: XServer 1: Writing X server authority to /var/run/lightdm/root/:1
[+0.02s] DEBUG: XServer 1: Launching X Server
[+0.02s] DEBUG: Launching process 5487: /usr/bin/X -core :1 -seat seat0 -auth /var/run/lightdm/root/:1 -nolisten tcp vt7 -novtswitch
[+0.02s] DEBUG: XServer 1: Waiting for ready signal from X server :1
[+0.02s] DEBUG: Acquired bus name org.freedesktop.DisplayManager
[+0.02s] DEBUG: Registering seat with bus path /org/freedesktop/DisplayManager/Seat0
[+0.02s] DEBUG: Loading users from org.freedesktop.Accounts
[+0.02s] DEBUG: User /org/freedesktop/Accounts/User1000 added
[+0.31s] DEBUG: Seat seat0 changes active session to c12
[+0.91s] DEBUG: Got signal 10 from process 5487
[+0.91s] DEBUG: XServer 1: Got signal from X server :1
[+0.91s] DEBUG: XServer 1: Connecting to XServer :1
[+1.18s] DEBUG: Seat seat0: Display server ready, starting session authentication
[+1.18s] DEBUG: Session pid=5642: Started with service 'lightdm-greeter', username 'lightdm'
[+1.18s] DEBUG: Seat seat0 changes active session to 
[+1.20s] DEBUG: Session pid=5642: Authentication complete with return value 0: Success
[+1.20s] DEBUG: Seat seat0: Session authenticated, running command
[+1.20s] DEBUG: Session pid=5642: Running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter
[+1.20s] DEBUG: Creating shared data directory /var/lib/lightdm-data/lightdm
[+1.20s] DEBUG: Session pid=5642: Logging to /var/log/lightdm/seat0-greeter.log
[+1.27s] DEBUG: Activating VT 7
[+1.28s] DEBUG: Activating login1 session c13
[+1.28s] DEBUG: Seat seat0 changes active session to c13
[+1.28s] DEBUG: Session c13 is already active
[+1.51s] DEBUG: Greeter connected version=1.22.0 api=1 resettable=false
[+1.79s] DEBUG: Greeter start authentication for hein
[+1.79s] DEBUG: Session pid=5796: Started with service 'lightdm', username 'hein'
[+1.80s] DEBUG: Session pid=5796: Got 1 message(s) from PAM
[+1.80s] DEBUG: Prompt greeter with 1 message(s)
[+3.52s] DEBUG: Seat seat0 changes active session to 2
[+29.98s] DEBUG: Greeter closed communication channel
[+29.98s] DEBUG: Session pid=5642: Exited with return value 0
[+29.98s] DEBUG: Seat seat0: Session stopped
[+29.98s] DEBUG: Seat seat0: Stopping; failed to start a greeter
[+29.98s] DEBUG: Seat seat0: Stopping
[+29.98s] DEBUG: Seat seat0: Stopping display server
[+29.98s] DEBUG: Sending signal 15 to process 5487
[+29.98s] DEBUG: Seat seat0: Stopping session
[+29.98s] DEBUG: Session pid=5796: Sending SIGTERM
[+29.98s] DEBUG: Session pid=5796: Terminated with signal 15
[+29.98s] DEBUG: Session: Failed during authentication
[+29.98s] DEBUG: Seat seat0: Session stopped
[+29.98s] DEBUG: Process 5487 exited with return value 0
[+29.98s] DEBUG: XServer 1: X server stopped
[+29.98s] DEBUG: Releasing VT 7
[+29.98s] DEBUG: XServer 1: Removing X server authority /var/run/lightdm/root/:1
[+29.98s] DEBUG: Seat seat0: Display server stopped
[+29.98s] DEBUG: Seat seat0: Stopped
[+29.98s] DEBUG: Required seat has stopped
[+29.98s] DEBUG: Stopping display manager
[+29.98s] DEBUG: Display manager stopped
[+29.98s] DEBUG: Stopping daemon
[+29.98s] DEBUG: Exiting with return value 1
[+0.00s] DEBUG: Logging to /var/log/lightdm/lightdm.log
[+0.00s] DEBUG: Starting Light Display Manager 1.22.0, UID=0 PID=5861
[+0.00s] DEBUG: Loading configuration dirs from /usr/share/lightdm/lightdm.conf.d
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/45-unity8.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-disable-guest.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-disable-log-backup.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-greeter-wrapper.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-guest-wrapper.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-unity-greeter.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-xserver-command.conf
[+0.00s] DEBUG: Loading configuration dirs from /usr/local/share/lightdm/lightdm.conf.d
[+0.00s] DEBUG: Loading configuration dirs from /etc/xdg/lightdm/lightdm.conf.d
[+0.00s] DEBUG: Loading configuration from /etc/lightdm/lightdm.conf
[+0.00s] DEBUG: Registered seat module local
[+0.00s] DEBUG: Registered seat module xremote
[+0.00s] DEBUG: Registered seat module unity
[+0.00s] DEBUG: Using D-Bus name org.freedesktop.DisplayManager
[+0.02s] DEBUG: Monitoring logind for seats
[+0.02s] DEBUG: New seat added from logind: seat0
[+0.02s] DEBUG: Seat seat0: Loading properties from config section Seat:*
[+0.02s] DEBUG: Seat seat0: Starting
[+0.02s] DEBUG: Seat seat0: Creating greeter session
[+0.02s] DEBUG: Seat seat0: Creating display server of type x
[+0.02s] DEBUG: Using VT 7
[+0.02s] DEBUG: Seat seat0: Starting local X display on VT 7
[+0.02s] DEBUG: XServer 0: Logging to /var/log/lightdm/x-0.log
[+0.02s] DEBUG: XServer 0: Writing X server authority to /var/run/lightdm/root/:0
[+0.02s] DEBUG: XServer 0: Launching X Server
[+0.02s] DEBUG: Launching process 5866: /usr/bin/X -core :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+0.02s] DEBUG: XServer 0: Waiting for ready signal from X server :0
[+0.02s] DEBUG: Acquired bus name org.freedesktop.DisplayManager
[+0.02s] DEBUG: Registering seat with bus path /org/freedesktop/DisplayManager/Seat0
[+0.02s] DEBUG: Loading users from org.freedesktop.Accounts
[+0.02s] DEBUG: User /org/freedesktop/Accounts/User1000 added
[+0.04s] DEBUG: Seat seat0 changes active session to c13
[+0.61s] DEBUG: Got signal 10 from process 5866
[+0.61s] DEBUG: XServer 0: Got signal from X server :0
[+0.61s] DEBUG: XServer 0: Connecting to XServer :0
[+0.66s] DEBUG: Seat seat0: Display server ready, starting session authentication
[+0.66s] DEBUG: Session pid=5888: Started with service 'lightdm-greeter', username 'lightdm'
[+0.68s] DEBUG: Session pid=5888: Authentication complete with return value 0: Success
[+0.68s] DEBUG: Seat seat0: Session authenticated, running command
[+0.68s] DEBUG: Session pid=5888: Running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter
[+0.68s] DEBUG: Creating shared data directory /var/lib/lightdm-data/lightdm
[+0.68s] DEBUG: Session pid=5888: Logging to /var/log/lightdm/seat0-greeter.log
[+0.71s] DEBUG: Activating VT 7
[+0.71s] DEBUG: Activating login1 session c14
[+0.71s] DEBUG: Seat seat0 changes active session to c14
[+0.71s] DEBUG: Session c14 is already active
[+0.78s] DEBUG: Greeter connected version=1.22.0 api=1 resettable=false
[+0.92s] DEBUG: Greeter start authentication for hein
[+0.92s] DEBUG: Session pid=5943: Started with service 'lightdm', username 'hein'
[+0.92s] DEBUG: Session pid=5943: Got 1 message(s) from PAM
[+0.92s] DEBUG: Prompt greeter with 1 message(s)
[+4.29s] DEBUG: Seat seat0 changes active session to 2
[+29.09s] DEBUG: Greeter closed communication channel
[+29.09s] DEBUG: Session pid=5888: Exited with return value 0
[+29.09s] DEBUG: Seat seat0: Session stopped
[+29.09s] DEBUG: Seat seat0: Stopping; failed to start a greeter
[+29.09s] DEBUG: Seat seat0: Stopping
[+29.09s] DEBUG: Seat seat0: Stopping display server
[+29.09s] DEBUG: Sending signal 15 to process 5866
[+29.09s] DEBUG: Seat seat0: Stopping session
[+29.09s] DEBUG: Session pid=5943: Sending SIGTERM
[+29.09s] DEBUG: Session pid=5943: Terminated with signal 15
[+29.09s] DEBUG: Session: Failed during authentication
[+29.09s] DEBUG: Seat seat0: Session stopped
[+29.09s] DEBUG: Process 5866 exited with return value 0
[+29.09s] DEBUG: XServer 0: X server stopped
[+29.09s] DEBUG: Releasing VT 7
[+29.09s] DEBUG: XServer 0: Removing X server authority /var/run/lightdm/root/:0
[+29.09s] DEBUG: Seat seat0: Display server stopped
[+29.09s] DEBUG: Seat seat0: Stopped
[+29.09s] DEBUG: Required seat has stopped
[+29.09s] DEBUG: Stopping display manager
[+29.09s] DEBUG: Display manager stopped
[+29.09s] DEBUG: Stopping daemon
[+29.09s] DEBUG: Exiting with return value 1
[+0.00s] DEBUG: Logging to /var/log/lightdm/lightdm.log
[+0.00s] DEBUG: Starting Light Display Manager 1.22.0, UID=0 PID=6033
[+0.00s] DEBUG: Loading configuration dirs from /usr/share/lightdm/lightdm.conf.d
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/45-unity8.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-disable-guest.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-disable-log-backup.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-greeter-wrapper.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-guest-wrapper.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-unity-greeter.conf
[+0.00s] DEBUG: Loading configuration from /usr/share/lightdm/lightdm.conf.d/50-xserver-command.conf
[+0.00s] DEBUG: Loading configuration dirs from /usr/local/share/lightdm/lightdm.conf.d
[+0.00s] DEBUG: Loading configuration dirs from /etc/xdg/lightdm/lightdm.conf.d
[+0.00s] DEBUG: Loading configuration from /etc/lightdm/lightdm.conf
[+0.00s] DEBUG: Registered seat module local
[+0.00s] DEBUG: Registered seat module xremote
[+0.00s] DEBUG: Registered seat module unity
[+0.00s] DEBUG: Using D-Bus name org.freedesktop.DisplayManager
[+0.02s] DEBUG: Monitoring logind for seats
[+0.02s] DEBUG: New seat added from logind: seat0
[+0.02s] DEBUG: Seat seat0: Loading properties from config section Seat:*
[+0.02s] DEBUG: Seat seat0: Starting
[+0.02s] DEBUG: Seat seat0: Creating greeter session
[+0.02s] DEBUG: Seat seat0: Creating display server of type x
[+0.02s] DEBUG: Using VT 7
[+0.02s] DEBUG: Seat seat0: Starting local X display on VT 7
[+0.02s] DEBUG: XServer 1: Logging to /var/log/lightdm/x-1.log
[+0.02s] DEBUG: XServer 1: Writing X server authority to /var/run/lightdm/root/:1
[+0.02s] DEBUG: XServer 1: Launching X Server
[+0.02s] DEBUG: Launching process 6038: /usr/bin/X -core :1 -seat seat0-auth /var/run/lightdm/root/:1 -nolisten tcp vt7 -novtswitch
[+0.02s] DEBUG: XServer 1: Waiting for ready signal from X server :1
[+0.02s] DEBUG: Acquired bus name org.freedesktop.DisplayManager
[+0.02s] DEBUG: Registering seat with bus path /org/freedesktop/DisplayManager/Seat0
[+0.02s] DEBUG: Loading users from org.freedesktop.Accounts
[+0.02s] DEBUG: User /org/freedesktop/Accounts/User1000 added
[+0.27s] DEBUG: Seat seat0 changes active session to c14
[+0.88s] DEBUG: Got signal 10 from process 6038
[+0.88s] DEBUG: XServer 1: Got signal from X server :1
[+0.88s] DEBUG: XServer 1: Connecting to XServer :1
[+1.14s] DEBUG: Seat seat0: Display server ready, starting session authentication
[+1.14s] DEBUG: Session pid=6193: Started with service 'lightdm-greeter', username 'lightdm'
[+1.14s] DEBUG: Seat seat0 changes active session to 
[+1.16s] DEBUG: Session pid=6193: Authentication complete with return value 0: Success
[+1.16s] DEBUG: Seat seat0: Session authenticated, running command
[+1.16s] DEBUG: Session pid=6193: Running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter
[+1.16s] DEBUG: Creating shared data directory /var/lib/lightdm-data/lightdm
[+1.16s] DEBUG: Session pid=6193: Logging to /var/log/lightdm/seat0-greeter.log
[+1.21s] DEBUG: Activating VT 7
[+1.21s] DEBUG: Activating login1 session c15
[+1.22s] DEBUG: Seat seat0 changes active session to c15
[+1.22s] DEBUG: Session c15 is already active
[+1.49s] DEBUG: Greeter connected version=1.22.0 api=1 resettable=false
[+1.75s] DEBUG: Greeter start authentication for hein
[+1.75s] DEBUG: Session pid=6340: Started with service 'lightdm', username 'hein'
[+1.75s] DEBUG: Session pid=6340: Got 1 message(s) from PAM
[+1.75s] DEBUG: Prompt greeter with 1 message(s)
[+6.23s] DEBUG: Seat seat0 changes active session to 2

Thanks in advance!

Have you tried reinstalling lightdm.
Try:
sudo apt-get install --reinstall lightdm

It kind looks as if the greeter got corrupted or something.

[+1.20s] DEBUG: Session pid=5642: Running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/unity-greeter

You may want to check that configuration which is in /usr/share/lightdm/lightdm.conf.d/. One of those files has the configuration for your greeter.