Eu fiz isso e funcionou:
root@gcomputer:~# apt-get purge lightdm
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following package was automatically installed and is no longer required:
liblept5
Use 'apt autoremove' to remove it.
The following packages will be REMOVED:
lightdm* lightdm-kde-greeter* ubuntu-desktop*
0 upgraded, 0 newly installed, 3 to remove and 1 not upgraded.
After this operation, 3152 kB disk space will be freed.
Do you want to continue? [Y/n]
(Reading database ... 499520 files and directories currently installed.)
Removing lightdm-kde-greeter (0.3.2.2-1ubuntu1) ...
Purging configuration files for lightdm-kde-greeter (0.3.2.2-1ubuntu1) ...
Removing ubuntu-desktop (1.361) ...
Removing lightdm (1.18.2-0ubuntu1) ...
Purging configuration files for lightdm (1.18.2-0ubuntu1) ...
insserv: warning: script 'S04powernowd.early' missing LSB tags and overrides
insserv: warning: script 'powernowd.early' missing LSB tags and overrides
insserv: warning: script 'S04powernowd.early' missing LSB tags and overrides
insserv: warning: script 'powernowd.early' missing LSB tags and overrides
dpkg: warning: while removing lightdm, directory '/var/lib/lightdm-data' not empty so not removed
dpkg: warning: while removing lightdm, directory '/etc/lightdm' not empty so not removed
Processing triggers for dbus (1.10.6-1ubuntu3) ...
Processing triggers for man-db (2.7.5-1) ...
root@gcomputer:~# rm -rf /etc/lightdm
root@gcomputer:~# rm -rf /var/lib/lightdm-data
root@gcomputer:~# apt-get install ubuntu-desktop
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
liblept5 liblightdm-qt-3-0
Use 'apt autoremove' to remove them.
The following additional packages will be installed:
lightdm
Suggested packages:
bindfs
The following NEW packages will be installed:
lightdm ubuntu-desktop
0 upgraded, 2 newly installed, 0 to remove and 1 not upgraded.
Need to get 120 kB of archives.
After this operation, 726 kB of additional disk space will be used.
Do you want to continue? [Y/n]
Get:1 http://de.archive.ubuntu.com/ubuntu xenial-updates/main amd64 lightdm amd64 1.18.2-0ubuntu1 [116 kB]
Get:2 http://de.archive.ubuntu.com/ubuntu xenial/main amd64 ubuntu-desktop amd64 1.361 [3616 B]
Fetched 120 kB in 0s (490 kB/s)
Preconfiguring packages ...
Selecting previously unselected package lightdm.
(Reading database ... 499338 files and directories currently installed.)
Preparing to unpack .../lightdm_1.18.2-0ubuntu1_amd64.deb ...
Unpacking lightdm (1.18.2-0ubuntu1) ...
Selecting previously unselected package ubuntu-desktop.
Preparing to unpack .../ubuntu-desktop_1.361_amd64.deb ...
Unpacking ubuntu-desktop (1.361) ...
Processing triggers for systemd (229-4ubuntu7) ...
Processing triggers for ureadahead (0.100.0-19) ...
ureadahead will be reprofiled on next reboot
Processing triggers for dbus (1.10.6-1ubuntu3) ...
Processing triggers for man-db (2.7.5-1) ...
Setting up lightdm (1.18.2-0ubuntu1) ...
insserv: warning: script 'S04powernowd.early' missing LSB tags and overrides
insserv: warning: script 'powernowd.early' missing LSB tags and overrides
Setting up ubuntu-desktop (1.361) ...
Processing triggers for dbus (1.10.6-1ubuntu3) ...
Processing triggers for systemd (229-4ubuntu7) ...
Processing triggers for ureadahead (0.100.0-19) ...
root@gcomputer:~# systemctl start lightdm