Ok, here the problem is:
The server never continues the conversation or creates the room.
It’s getting a little bit confusing, because in your first post you said, the invitee could not join the room, after he has received the invitation, (because the server returned an error in the presence stanza, likely because the room was locked)
Now the server just doesn’t respond at all, when the owner creates the room.
Did you check the server logs for anything suspicous?
I could not reproduce it with Adium (1.5.9). I was able to create a room and finish default configuration and join it. I could also invite others, which were able to join then.
Btw: Where can I find the XMPP traffic/logs in Adium?