Check the water level device is working... This indicates an overfill.
The water level device usually takes the form of a pressure switch and it has a thin tube going to it.. If the tube gets blocked then the device can not sense the water that has entered the appliance and therefore continues to fill.. and as in your case, an overfill occurs causing water to exit the tub and activate the safety device.
The water pressure device on dishwashers can normally be found at the lower part of the appliance by removing the right hand side panel.
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