There seems to be a problem with xfce's system tray, you can use another
system tray; summon the terminal emulator and:
$ sudo aptitude install xfce4-indicator-plugin
Then, go to the panel, left click and select panel and add elements to the
panel. After that, select the new indicator you have downloaded. You will see
that the indicator is "too complete", press right mouse button with the
cursor over the new tray and select properties to be able to choose what
applications you don't want to show up. After selecting what you want,
restart the session.
Tell us if that worked.
