water container full light comes on when empty?

my miele condenser dryer has developed a problem. The water collection light comes on within minutes of starting the dryer (obviously, I have emptied the container). I have cleaned the condenser unit, I have taken the hose out and put it in a bucket to collect but the problem persists. I have cleaned everything I can think of or find. The dryer is 4 years old and has only ever been used a couple of times a week and only during the winter so it is not old in those terms.