Recently I upgraded from Ubuntu 17.10 to Ubuntu 18.04, since then, when I login after reboot I get the default ubuntu login screen, and when I login after lock/sleep I get gnome login screen (Small rectangle with password field)
I want to get the default login screen after lock too...
I've searched the internet and I found a similar issue but on ubuntu 15.*, and the solution didn't work
Same issue. Managed to fix it by uninstalling GDM3, and everything seems to still work fine, but I'm not a Linux expert