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)
Definitely can. But AUR make it easier, once it's not officially supported and has permission to install things on your base system with root access.
It's kinda the same as you download a deb file and install it or use ppa on debian-based distro. All of that can make your system breaks, but in other distros the general recommendation is to use official repos always when the app is available on repos
Right -- so like I said, nothing to do with the AUR and everything to do with being smart about what software you install.