screen freezes when I try to log

[BELAHCENE Abdelkader <abdelkader.belahcene@enst.dz>]

Hi,
Sometimes the login thru the lightdm fails. That means I log but the screen freezes and nothing happens, no login no desktop ( using the xfce4).
When I go to the terminal ( using   CTRL+ALT+F1 ), and I log using shell,  I can start the : startx -- :1,  and access to the graphical Desktop..
Where is the problem?  in lightdm or xfce4 ??
I install lxde  and want to log with lxde,  unfortunatly, there is no menu  in the lighdm login,  to choose the manager.
I am using linuxLite 3.8
regards