I have cleaned the drain tube, as described in the other answers, several times successfully, but found that eventually it has no effect - water still comes out of the vent. My wife came up with the solution:
First clean the tube as described in the other answers.
Reconnect the bottom of the drain tube. Open the flap on the salt dispenser. Take an empty Jif lemon and fill with water, attach it to the top of the drain tube (the end nearest the fan) - take care not to overstretch the mouth of the tube. Then give an almighty squeeze on the Jif plastic bulb. This forces water through the box at the bottom end of the tube and expels any collected gunk out of a small hole in the salt dispenser.
Dave B, Didcot, Oxon, October 2010