Comment 1 for bug 1453303

Workaround:
Open "/etc/xdg/autostart/hp-systray.desktop" in a text editor.
Change line 7 from:
    Exec=hp-systray -x
to:
    Exec=sleep 15 && hp-systray -x

You may need to adjust the amount of sleep time depending on the speed of your PC.
This means that hp-systray will not actually start until well after the system tray is available, thus avoiding whatever bug is causing the problem.