After an aptitude safe-upgrade my laptop doesn't boot any more.I recorded this video of the boot process.I have access to tty1, and from there I can run startx which works without a problem.Here is the output:X.Org X Server 1.17.2Release Date: 2015-06-16X Protocol Version 11, Revision 0Build Operating System: Linux 4.1.0 x86_64 DebianCurrent Operating System: Linux debian 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt11-1 (2015-05-24) x86_64Kernel command line: BOOT_IMAGE=/vmlinuz-3.16.0-4-amd64 root=/dev/mapper/debian--vg-root ro quietBuild Date: 11 August 2015  10:51:15AMxorg-server 2:1.17.2-1.1 (http://www.debian.org/support)Current version of pixman: 0.32.6   Before reporting problems, check http://wiki.x.org   to make sure that you have the latest version.Markers: (--) probed, (**) from config file, (==) default setting,   (++) from command line, (!!) notice, (II) informational,   (WW) warning, (EE) error, (NI) not implemented, (??) unknown.(==) Log file: /var/log/Xorg.2.log, Time: Fri Aug 28 15:55:04 2015(==) Using system config directory /usr/share/X11/xorg.conf.dI thought that the problem could be de DM. Here is /var/lightdm/lightdm.log output:[+0.00s] DEBUG: Logging to /var/log/lightdm/lightdm.log[+0.00s] DEBUG: Starting Light Display Manager 1.14.2, UID=0 PID=1275[+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/01_debian.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: Using D-Bus name org.freedesktop.DisplayManager[+0.00s] DEBUG: Registered seat module xlocal[+0.00s] DEBUG: Registered seat module xremote[+0.00s] DEBUG: Registered seat module unity[+0.01s] DEBUG: Monitoring logind for seats[+0.01s] DEBUG: New seat added from logind: seat0[+0.01s] DEBUG: Loading properties from config section SeatDefaults[+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[+0.01s] DEBUG: Could not run plymouth --ping: Failed to execute child process plymouth (No such file or directory)[+0.01s] DEBUG: Using VT 7[+0.01s] DEBUG: Seat seat0: Starting local X display on VT 7[+0.01s] DEBUG: DisplayServer x-0: Logging to /var/log/lightdm/x-0.log[+0.01s] DEBUG: DisplayServer x-0: Writing X server authority to /var/run/lightdm/root/:0[+0.01s] DEBUG: DisplayServer x-0: Launching X Server[+0.01s] DEBUG: Launching process 1280: /usr/bin/X :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch[+0.01s] DEBUG: DisplayServer x-0: Waiting for ready signal from X server :0[+0.01s] DEBUG: Acquired bus name org.freedesktop.DisplayManager[+0.01s] DEBUG: Registering seat with bus path /org/freedesktop/DisplayManager/Seat0[+0.01s] DEBUG: Loading users from org.freedesktop.Accounts[+0.02s] DEBUG: User /org/freedesktop/Accounts/User1000 added[+0.35s] DEBUG: Got signal 10 from process 1280[+0.35s] DEBUG: DisplayServer x-0: Got signal from X server :0[+0.35s] DEBUG: DisplayServer x-0: Connecting to XServer :0[+0.35s] DEBUG: Seat seat0: Display server ready, starting session authentication[+0.35s] DEBUG: Session pid=1291: Started with service 'lightdm-greeter', username 'lightdm'[+0.37s] DEBUG: Session pid=1291: Authentication complete with return value 0: Success[+0.37s] DEBUG: Seat seat0: Session authenticated, running command[+0.37s] DEBUG: Session pid=1291: Running command /usr/sbin/lightdm-gtk-greeter[+0.37s] DEBUG: Creating shared data directory /var/lib/lightdm/data/lightdm[+0.37s] DEBUG: Session pid=1291: Logging to /var/log/lightdm/x-0-greeter.log[+0.42s] DEBUG: Activating VT 7[+0.42s] DEBUG: Activating login1 session c7[+0.42s] DEBUG: Seat seat0 changes active session to c7[+0.42s] DEBUG: Session c7 is already active[+0.50s] DEBUG: Session pid=1291: Greeter closed communication channel[+0.50s] DEBUG: Session pid=1291: Exited with return value 0[+0.50s] DEBUG: Seat seat0: Session stopped[+0.50s] DEBUG: Seat seat0: Stopping; failed to start a greeter[+0.50s] DEBUG: Seat seat0: Stopping[+0.50s] DEBUG: Seat seat0: Stopping display server[+0.50s] DEBUG: Sending signal 15 to process 1280[+0.51s] DEBUG: Seat seat0 changes active session to[+0.51s] CRITICAL: session_get_login1_session_id: assertion 'session != NULL' failed[+1.52s] DEBUG: Process 1280 exited with return value 0[+1.52s] DEBUG: DisplayServer x-0: X server stopped[+1.52s] DEBUG: Releasing VT 7[+1.52s] DEBUG: DisplayServer x-0: Removing X server authority /var/run/lightdm/root/:0[+1.52s] DEBUG: Seat seat0: Display server stopped[+1.52s] DEBUG: Seat seat0: Stopped[+1.52s] DEBUG: Required seat has stopped[+1.52s] DEBUG: Stopping display manager[+1.52s] DEBUG: Display manager stopped[+1.52s] DEBUG: Stopping daemon[+1.52s] DEBUG: Exiting with return value 1Notice the following line?[+0.51s] CRITICAL: session_get_login1_session_id: assertion 'session != NULL' failedMate uses Lightdm, so I removed and installed again, but that didn't make any difference.Then I installed XLDE, which uses lxdm. Again, same problem: flashing screen and blinking coursor.So it's nothing to do with X, because I can startx manually, and it's nothing to do with the desktop environment, as I have the same problem with both Mate and LXDE. Anyone have a suggestion where should I begin to fix this problem?
Flashing screen and blinking underscore at boot
debian