The server implementation (v .3.6.4) does not correctly reply to iq requests, causing converting one-on-one chats into multi user chat not working for some clients.
The reason is that when the client sends the following request (request unique room) the client never gets a reply, while according to spec the server implementation MUST reply with a not supported (or similar) reply.
To fix the issue, the server must reply with some kind of iq error reply, or even bette, support the unique room id generation feature - according to http://xmpp.org/extensions/xep-0045.html#continue