EDIT: I kinda solved it by installing Wayland (with Nvidia card, Ouch!) to replace Xorg. Not sure if this is gonna last though. Perhaps Manjaro is the one I'm gonna throw out FIRST if anything happens from now on.
What should be the first line of defense? Timeshift?
This happened after I installed AUR package masterpdfeditor and 2 applications from github (some hashing algorithm programs, I think they were "Dilithium" and "Latice-based-cryptography-main", one of them was provided by NIST.)
If using GUI: I login, black screen for few seconds, then back at login screen.
If going to ctrl+alt+f2, login successful, then startx, see picture provided (higher quality).
I tried adding a new user, but result is the same.
I have a live usb to do the Timeshift. (I can also chroot if necessary... But I'm not extremely professional)
Have you checked the log files under /var/log/xorg.0....log?
Also see https://wiki.archlinux.org/title/Xorg#Troubleshooting
If you want to get your graphical session back quickly, maybe try setting up GNOME with wayland (https://wiki.archlinux.org/title/GNOME#Wayland_sessions)
You can also check /var/log/pacman.log for the packages you installed, then pacman -Ql the packages to list what files they might have changed