Invisible Host (room) is a Server Bug that makes a recently-made host room to become invisible to the rest of the players.
At the moment, there is not any known cause for this bug. It seems to happen only the first time a player hosts a room, and only once per player. It is often associated to a limited bandwith, but there is no evidence for this.
Because of this bug, the rest of the players cannot see or enter the affected host room, and the hosts don't even notice their host rooms are buggy.
The only way for the host to solve this bug is to abort the current host room and to make a new one, which surely won't have the same problem.