Hi,
1. Please try to reconfigure your tray widget (right-click on an arrow on its left side, then choose "Configure "System Tray""). Just re-enable tray icon on the "Items" page.
2. If this does not help, please try to log out then switch to another console (Ctrl+Alt+F2). Log in, then move ~/.kde/share/config to other folder (mv ~/.kde/share/config ~/kdeconfigs). Restart your computer. Try to restore the needed configuration by moving files of the corresponding applications to the recreated folder, one-by-one.
Hope this helps.
Best regards,
Yuri