What I meant was the JID of the room, for clients connecting only by JIDs. (for what I understood, it is outpost2@conference.outpost2.net)
As for my problem, here it is:
Sent a join request to <outpost2@conference.outpost2.net>...
Received error packet [remote-server-not-found] from <outpost2@conference.outpost2.net/Lukc>
Error message from server: Server-to-server connection failed: DNS resolution failed
It may be linked to the missing messages TH300 is experiencing, but would be very strange. I’m not sure what could be the problem atm. :/