Good tips, Dan. As said above, there is a ball and spring mechanism that is part of the filter basket. If you still experience the problem after you clean it, they do offer new filter baskets online that come with the ball and spring mechanism. Here is a link to the troubleshooting page that has a guide which details how to replace the filter basket and also has links to websites that offer the filter basket replacement part.
Braun Aromaster KF 440 Troubleshooting
This could also occur due to overflowing. If too much water was poured into the reservoir, the filter basket could overflow if the carafe is removed for too long.