Comment # 6 on bug 1214043 from Michael Pujos

Mystery solved.

The problem was caused by the xdg-desktop-portal-gnome package being
*installed*. Uninstalling it fixed the issue.

This Arch forum thread revealed that fix:
https://bbs.archlinux.org/viewtopic.php?id=285590

Longer story:

before stumbling on the post above, I noticed that
xdg-desktop-portal-gnome.service did not start:


Aug 25 12:02:39 p72 systemd[6676]: Dependency failed for Portal service (GNOME
implementation).
Aug 25 12:02:39 p72 systemd[6676]: xdg-desktop-portal-gnome.service: Job
xdg-desktop-portal-gnome.service/start failed with result 'dependency'.

The cause is that it depends on graphical-session.target

[Unit]
Description=Portal service (GNOME implementation)
After=graphical-session.target
Requisite=graphical-session.target
PartOf=graphical-session.target

And I start my system with startx (launched automatically after login on
vconsole, so without a Desktop Manager), thus it is in multi-user.target and
xdg-desktop-portal-gnome.service cannot start.

The xdg-desktop-portal-gnome package was installed automatically on my system
during one the latest snapshot a few days ago, probably as an optional
dependency for something else.

So the permanent fix on my system (and other weird people not using a Desktop
Manager) is to lock package xdg-desktop-portal-gnome so it is never installed.
That also explains why very few user had this issue.