In libxfce4panel/xfce-panel-plugin.c change image = gtk_image_new_from_icon_name ("gtk-help", GTK_ICON_SIZE_MENU); to image = gtk_image_new_from_icon_name ("help-browser", GTK_ICON_SIZE_MENU);
Forgot to append the bug id, fixed by https://git.xfce.org/xfce/xfce4-panel/commit/?id=3ca9e8b216a78ca64ebbf1bef7fc2043be6b1c93 https://git.xfce.org/xfce/xfce4-panel/commit/?id=3c48132257b6b921d81d64aa3de0f91e22995d0f