Comment # 9 on bug 1089287 from Stefan Dirsch

(In reply to Dr. Werner Fink from comment #7)
>       [ 18039.023] (EE) systemd-logind: failed to get session: PID 31661
> does not belong to any known session
> 
> ... in other words this option does not work as it should ... maybe
> something has been disabled in the X server which does not do the
> controlling terminal stuff.

No idea. Andreas Jaeger ran into the same issue after updating his system from
42.3 to 15.0 (forgot about bugzilla number). He didn't run into the issue when
installing 42.3 from scratch and then updating his sytem to 15.0. So not so
easy to reproduce ...

Could be some dbus related issue ...

Needs to be debugged on the system.

hw/xfree86/os-support/linux/systemd-logind.c

static void
connect_hook(DBusConnection *connection, void *data)
[...]
    msg = dbus_message_new_method_call("org.freedesktop.login1",
            "/org/freedesktop/login1", "org.freedesktop.login1.Manager",
            "GetSessionByPID");
[...]
    reply = dbus_connection_send_with_reply_and_block(connection, msg,
                                                      DBUS_TIMEOUT_USE_DEFAULT,
&error);
    if (!reply) {
        LogMessage(X_ERROR, "systemd-logind: failed to get session: %s\n",
                   error.message);
[...]

Maybe somebody here known how to use dbus-send, so things could be investigated
on the system even without debugging this ...

> Btw: Without the option an other error messages appears which shouts that
> -keeptty option is required

This message has been added with git commit of comment#8.