Display brightness doesn't work

[GGORG0]

The brightness widget in Waybar and the light utility don't work. The brightness reported by the Waybar widget is always 50% and light reports it at 49.75%.
When I try scrolling on the Waybar widget, it spams the journal with:

sty 24 16:26:17 ggorg-elitebook .waybar-wrapped[2173]:
                                                       unhandled exception (type Glib::Error) in signal handler:
                                                       domain: g-dbus-error-quark
                                                       code  : 41
                                                       what  : GDBus.Error:org.freedesktop.DBus.Error.UnknownObject: Unknown object '/org/freedesktop/login1/session/self'.

and when running light:

~
$ light
49.75

$ light -S 1

$ echo $?
1