delating transport resulted with delating my account
I received a message from server admin (it was about new transports available on my jabber server). New contact "not in the roster" appeared in my roster in Transports group. I delated it with context menu This resulted with delating my account with all my contacts. The admin message is send automatically to every new user, so I tested it few times (by registering new account and delating it in a way described above), but only on jabberpl.org sever, so I'm not sure if it is Gajim's problem or maybe the server.