Gajim locks up if roster is changed externally
I reported this problem some days ago in the Gajim chatroom but I think opening a ticket would be a good idea though.
I played around with ejabberd web interface and there you can change the roster of your users (like adding and deleting entries). Gajim doesn't like this. I could still read messages but the interface was completely unresponsive, only system tray icon worked.