I've just tried waiting longer than 15 minutes but the lightdm screen didn't appear. Just the frozen mouse cursor on black background.
Also tried switching to console (Ctrl+Alt+F2 right after boot) after setup was done, but nothing happened. It would just go again to the mouse cursor on black background.
Also tried this key combination when the mouse coursor is on screen but no luck, as I didn't know when to input the Ctrl+Alt+F2 combination.
Anyway, I've had the idea of using putty on my desktop PC and succeeded to access the M3's console.
There I've typed sudo systemctl enable lightdm and rebooted. This worked.
Thanks, I can now try the KDE!