Starting with `libxcrypt` 4.4.21, weak password hashes (such as *MD5* and *SHA1*) are no longer accepted for new passwords. Users that still have their passwords stored with a weak hash will be asked to update their password on their next login.

If the login just fails (for example from display manager) switch to a virtual terminal (*Ctrl-Alt-F2*) and log in there once.

URL: https://archlinux.org/news/sorting-out-old-password-hashes/