Created attachment 3327 Screenshot of the issue When the panel starts for the first time, I have a black rectangle at the right extremity. It is not an icon, it's just some empty space with a black background which has the same size as the other icons. Restarting the panel fixes the issue.
Could you attach the out of the panel with "PANEL_DEBUG=systray xfce4-panel"
Same black rectangle is also in bug #7057.
Created attachment 3330 Log with PANEL_DEBUG=systray
Seems to be the same issue Christoph has. *** This bug has been marked as a duplicate of bug 7057 ***