The suggestion from https://bugs.launchpad.net/ubuntu/+source/lxpanel/+bug/1308348/comments/44 (disabling Indicator Application service from "Session and Startup"->"Application Autostart") made the icons appear again. Looks like a bug with integration between XFCE4 tray and Indicator-enabled application notifications.
The suggestion from https:/ /bugs.launchpad .net/ubuntu/ +source/ lxpanel/ +bug/1308348/ comments/ 44 (disabling Indicator Application service from "Session and Startup" ->"Application Autostart") made the icons appear again. Looks like a bug with integration between XFCE4 tray and Indicator-enabled application notifications.