When Ubuntu's "messaging menu" is removed, Liferea minimizes/closes to... nowhere. It's still running, but cannot be brought back. No icon appears in the systray.
Solution: When messaging menu is unavailable, minimize to system tray.
Even though restarting liferea did not work, restarting the computer entirely did. Don't know whether to close this ticket as "Invalid" or not.
The next unstable release 1.9.5 switches to GtkApplication instead of libunique. This allows a new mechanism of bringing forth the application after it has hidden (for whatever reason). Please retest with either git master (or 1.9.5)