After upgrading to Xubuntu 16.04, I found that the dropbox icon stopped working. The icon is black with a red slashed zero. I cannot click on it to bring up the dropbox menu.
The reason is that Dropbox starts to use “indicator area” for their tray icon instead of “notification area”. The problem can be fixed by running
$ dropbox stop && DBUS_SESSION_BUS_ADDRESS="" dropbox start
$ dropbox stop && dbus-launch dropbox start