Thank to Eric Dubois from ArcoLinux!
From ArcoLinux website: Fix for failed to start light display manager on 06/02/2019
February 6th 2019 we get an update in that breaks our installation.
I had seen what packages were installed and I did notice lightdm in there.
LIGHTDM is one of your major packages – it starts out display or login manager. Other important packages to keep track off is linux and systemd.
IF you did not see the packages that came in you can use our alias RIP in the terminal to see the last packages.
You computer will function fine until you reboot and then lightdm throws you lots of errors.
Get to a TTY with CTRL + ALT + F2 or F3 or … (Virtualbox users use RIGHT CTRL + F2
Choose the prior version with a number
Add lightdm to IgnorePkdg : Y
reboot and have fun again
Other Arch Linux users need to install downgrade with an AUR helper like yay or trizen.
A bug have already been opened.
EDIT: This has been fixed in 1:1.28.0-3.