Answer: empty the water heater to clean the gasket and the resistance.
First possibility, contact a professional who will do it for you. It's easier, but it can be expensive. Otherwise, you can do it yourself directly. In this case, first cut off the electricity supply and the water supply to your water heater, then empty the appliance. To avoid wasting too much water, use the water normally for your personal use.When the water heater is empty, you will know when you hear a hissing sound, indicating that the air has replaced the water in your cumulus. You will be able to thus open your cumulus, and clean the joint and the resistance by rubbing them with heated white vinegar or a specialized product which one finds in the trade. Take this opportunity to check if any of these need to be changed. Once you're done, reconnect everything!