I’m going to accept the fact of login using TTY instead of graphic greeter.
I also tried to uninstall again lightdm and lightdm-greeter and delete his directories (I discovered that are reported in /usr/lib/tmpfiles.d/lightdm.conf)
/var/lib/lightdm
/var/lib/lightdm-data
/run/lightdm
/var/cache/lightdm
Then reboot for the umpteenth time, reinstall it again and so on: lightdm doesn’t want to autostart anymore, and I repeat: I also restored the system backup from the day before the stable update.
I also disabled the service lightdm.service: is useless, since it doesn’t autostart anymore: is a weight on the startup.
So I login from TTY, by executing startxfce4, and I had to replicate the environment variables from ~/.profile to ~/.bashrc (eg, like export QT_QPA_PLATFORMTHEME="qt5ct");
This because seems that by starting the graphical environment from startxfce4 doesn’t source ~/.profile: I verified this by doing echo $QT_QPA_PLATFORMTHEME: resulted in an emtpy output, is ok instead by having the env variables in ~/.bashrc.
still in ~/.bashrc, I made an alias to quickly execute startxfce4:
alias sd="startxfce4" (sd stands for Start Desktop).
lxdm, the alternative for lightdm, works fine, but is not customizable like lightdm, and furthermore, the system’s mouse theme is not applyed by lxdm.