If the overheating protection trips, it is usually because the circulation pump sucks in air and the heater can switch on and off and then also activate the overheating protection. Is there enough water in the barrel?
It is possible to check the function of the heater.
That the overheating protection trips can also be due to the flow rocker being too close to the button on the microswitch so that it does not have time to "swing back" when the pump switches off and then the overheating is triggered. If it is, you will hear a "boiling sound" from the heater before it cuts out.
Check the last 2 pictures and where pictures 3-4 shows how the "flow rocker" is pushed to the left against the microswitch to activate it. If it is too close from the knob on the microswitch, it must instead be pushed in the other direction so that it comes further away. It probably has to be done when the pump is running so that there will be some resistance to push against. It is nevertheless important that after adjustment it is checked that the microswitch is activated when the pump is running and switches off when it stops.
Here is an animated video showing how the heater works
Here you can also see how it works in reality