Much higher energy use, maybe due to an old electrical heater?
So my thought was, could it be that this year for example one rod in the electrical heater corroded which lead to higher restistance => More power draw?


No. Higher resistance will reduce current draw. Thought experiment: disconnect the heater completely to create infinite resistance. Will the power consumed increase or decrease?

It's time for an energy audit. You can start by finding your energy meter and taking hourly readings or time how long it takes for the meter in increment by one or ten units. If these are consistent then you can switch off each circuit in turn and measure the change in timing.

Alternately you can get a power analyser with voltage and current probes and take measurements and log data for each load.