Hello. I hope to have the solution.
The problem is in /usr/share/xsessions/lightdm-xsession.desktop. The key "Exec=default" should be replaced for "Exec=startxfce4", for example. Now, the default session will be XFCE. For other window-managers I don't know the key value, but you can search this value in another file *.desktop into /usr/share/xsessions/.
