I was looking through my .pacnew files and applied some changes for the first time in years. I don’t have a comprehensive list of all config files, but it definitely included sddm among others. I did the 2025-12-15 stable update a few days earlier and that worked before applying the .pacnew files.
Now, sddm rejects my login with “Login failed” despite my definitely using the correct password (it worked before applying the .pacnew files). Using Ctrl+Alt+F3 to open tty3 and trying to login with the same credentials results with “Login incorrect”.
It is possible one of the .pacnew files was for /etc/passwd and that you have overwritten your passwd file. If you can log-in as root (or access the files by booting from a USB) you could check if there is a /etc/passwd-, which is a backup of the passwd file.
If you decide to try to revert to the backup copy, I would proceed cautiously, copying the current /etc/passwd somewhere safe before comparing the passwd and passwd- files, and possibly replacing passwd with the contents of passwd-.
revert those changes (if you can)
and then start over
you probably made a mistake or two when merging the new defaults with your working configuration
(that is what .pacnew files are: the new default settings
you can’t just replace what you have with defaults
… not without possibly show stopping consequences)
You should be able to track and see what files you have worked on
by looking into your shell history.
Knowing that would narrow the issue down quite a bit
if you yourself can’t remember on which files you worked.
I was able to fix this problem by using the /etc/pam.d/sddm and /etc/shells files (which I now remember that I have overwritten them with their .pacnew variants) from another working Manjaro system, where I have not used any .pacnew files before. I also re-set my user password to the correct password with passwd <user> for good measure, but I do not think that this made a difference. I do not know which of these changes were the actual fix, but I presume, that allowing /usr/bin/zsh in /etc/shells fixed it, because that is my default shell.
The only question is whether I have basically deployed a time bomb in my system, because I also manually merged /etc/default/grub and /etc/mkinitcpio.conf with their corresponding .pacnew files (instead of only viewing the diff and overwriting them with their .pacnew like I did with the other .pacnew files).
That one was indeed a problem causing failure to login when said .pacnew was issued.
It is better to manually merge the files than to blindly overwrite them with the .pacnew.
The .pacnew is only advisory, as per upstream’s defaults. Blindly replacing your existing configuration files with their respective .pacnew is a recipe for disaster.