Symptom
When running xfce4, I found that in the notifications area, the nm-applet (the applet that shows the network status) would sometimes take a while to arrive, and sometimes it would hang and not allow me to view or change network.
When I started xfce4-notifyd-config I noticed that the xfce4-notifyd daemon was not running.
Solution
It turned out that I had some kde utilities installed, and one of these, the org.kde.plasma.Notifications.service, was conflicting with the org.xfce.xfce4-notifyd.Notifications.service. Both notification services failed.
To solve the problem, simply remove /usr/share/dbus-1/services/org.kde.plasma.Notifications.service (ensuring first that /usr/share/dbus-1/services/org.xfce.xfce4-notifyd.Notifications.service exists).
Now log out and log in again and you should find that the nm-applet works and that when you start xfce4-notifyd-config, you don’t get the message that the daemon has not started.