xfce4-statusnotifier-plugin provides a panel area for status notifier items (application indicators). Applications may use these items to display their status and interact with user. This technology is a modern alternative to systray and follows the freedesktop.org specification regarding status notification.

There is already a running service which works with status notifier items. Most probably it's the indicator-application service which is used by xfce4-indicator-plugin. Make sure this service is not running: remove this indicator from xfce4-indicator-plugin and remove the service from autostart.