I've some new info. I installed 15.10 in server mode, installed lubuntu-desktop and ubuntu-desktop and started X without problem.
startx unity or startx both works.

But nor lightdm nor gdm start. lightdm crash and gdm gives a black screen.



systemctl status lightdm.service
● lightdm.service - Light Display Manager
   Loaded: loaded (/lib/systemd/system/lightdm.service; static; vendor preset: enabled)
   Active: failed (Result: start-limit) since Wed 2016-04-06 21:02:21 CEST; 7s ago
     Docs: man:lightdm(1)
  Process: 11960 ExecStartPre=/bin/sh -c [ "$(basename $(cat /etc/X11/default-display-manager 2>/dev/null))" = "lightdm" ] (code=exited, status=1/FAILURE)
 Main PID: 8131 (code=exited, status=1/FAILURE)

Apr 06 21:02:20 i27 systemd[1]: Failed to start Light Display Manager.
Apr 06 21:02:20 i27 systemd[1]: lightdm.service: Unit entered failed state.
Apr 06 21:02:20 i27 systemd[1]: lightdm.service: Failed with result 'exit-code'.
Apr 06 21:02:21 i27 systemd[1]: lightdm.service: Service hold-off time over, scheduling restart.
Apr 06 21:02:21 i27 systemd[1]: Stopped Light Display Manager.
Apr 06 21:02:21 i27 systemd[1]: lightdm.service: Start request repeated too quickly.
Apr 06 21:02:21 i27 systemd[1]: Failed to start Light Display Manager.
Apr 06 21:02:21 i27 systemd[1]: lightdm.service: Unit entered failed state.
Apr 06 21:02:21 i27 systemd[1]: lightdm.service: Failed with result 'start-limit'.
n@i27:~$ sudo  journalctl -xe
-- 
-- Unit lightdm.service has begun starting up.
Apr 06 21:02:20 i27 systemd[1]: lightdm.service: Control process exited, code=exited status=1
Apr 06 21:02:20 i27 systemd[1]: Failed to start Light Display Manager.
-- Subject: Unit lightdm.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit lightdm.service has failed.
-- 
-- The result is failed.
Apr 06 21:02:20 i27 systemd[1]: lightdm.service: Unit entered failed state.
Apr 06 21:02:20 i27 systemd[1]: lightdm.service: Failed with result 'exit-code'.
Apr 06 21:02:20 i27 systemd[1]: lightdm.service: Service hold-off time over, scheduling restart.
Apr 06 21:02:20 i27 systemd[1]: friendly-recovery.service: Cannot add dependency job, ignoring: Unit friendly-recovery.service i
Apr 06 21:02:20 i27 systemd[1]: Stopped Light Display Manager.
-- Subject: Unit lightdm.service has finished shutting down
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit lightdm.service has finished shutting down.
Apr 06 21:02:20 i27 systemd[1]: Starting Light Display Manager...
-- Subject: Unit lightdm.service has begun start-up
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit lightdm.service has begun starting up.
Apr 06 21:02:20 i27 systemd[1]: lightdm.service: Control process exited, code=exited status=1
Apr 06 21:02:20 i27 systemd[1]: Failed to start Light Display Manager.
-- Subject: Unit lightdm.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit lightdm.service has failed.
-- 
-- The result is failed.
Apr 06 21:02:20 i27 systemd[1]: lightdm.service: Unit entered failed state.
Apr 06 21:02:20 i27 systemd[1]: lightdm.service: Failed with result 'exit-code'.
Apr 06 21:02:21 i27 systemd[1]: lightdm.service: Service hold-off time over, scheduling restart.
Apr 06 21:02:21 i27 systemd[1]: friendly-recovery.service: Cannot add dependency job, ignoring: Unit friendly-recovery.service i
Apr 06 21:02:21 i27 systemd[1]: Stopped Light Display Manager.
-- Subject: Unit lightdm.service has finished shutting down
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit lightdm.service has finished shutting down.
Apr 06 21:02:21 i27 systemd[1]: lightdm.service: Start request repeated too quickly.
Apr 06 21:02:21 i27 systemd[1]: Failed to start Light Display Manager.
-- Subject: Unit lightdm.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit lightdm.service has failed.
-- 
-- The result is failed.
Apr 06 21:02:21 i27 systemd[1]: lightdm.service: Unit entered failed state.
Apr 06 21:02:21 i27 systemd[1]: lightdm.service: Failed with result 'start-limit'.