System tray icon doesn't reappear after explorer restart
If for whatever reason, your Windows Explorer process dies, then gajim quite correctly continues to run. However, it doesn't re-register it's icon in the system tray, so if minimised when the crash occurs, you lose all ability to access it, forcing you to use the process manager to kill the process and start a new one.
Gajim should re-register it's icon when a new Explorer window opens.