I've followed this YouTube video for my kettle (a Nefit EcomLine) and set the tap dial to it's lowest setting. Als the other settings in case you may wonder. But the water still comes almost boiling from the tap. With the current gas prices, I'd like it to not heat the water to such temperatures. What can be the cause?
If your water heater heats to a higher temperature than you have set it to, then the water heater is broken.
The thermostat could have fallen out of position, become disconnected, or otherwise broken. The thermostat itself should be decently cheap. If it is not working properly and it is not under warranty, then you can do no damage because a broken thing cannot be broke.
I gave Mazura a thumbs up for suggesting the installation of a mixing valve after the heater. It's a great idea.
This is a common-sense answer given instructions in a language I do not understand. You proclaimed that you have properly adjusted the setting, but it does not respond properly. Therefore, I respond: It is broken. In this case, the thermostat is not working properly.
First and foremost, check with your health office about the impact of kettle temperature on legionella pneumophilia and other bacteria growth, which vary by region. The water misadventures in Flint, Michigan revealed a heretofore little-understood problem: Too low a water heater (kettle) temperature turns it into a breeding ground for bacteria.
The bacteria can spread in aerosol fashion, in fact that is how it killed 33 at the American Legion convention in Philadelphia (hence its name). So having bacteria-laden water in a shower is a terrible idea.
So see what your health office has for advice, and follow it.
Note the the "stop bacteria" best practice, keeping the kettle at 60C, conflicts with the past best practice of keeping it at 40-45C to save energy. It also introduces a new complication: 60C water can scald, so should be used with blending faucets with safety thermostats (e.g. most modern 1-handle faucets already do this).
Scalding water should not be available to you
The only exception I can think is a special auxiliary tap fed by a separate, under-sink miniature kettle, typically 1-2 liters. These taps are specifically for hot water for tea. However, such heaters are always electric, as they are far too small for methane/propane to be practicable.
I worked on a hydronic system once; it had a boiler that only went down to 175 F or so.
The potable side of the system had a failed thermostatic mixing valve so the client had it set too low to do its other jobs efficiently (side walk melting, radiators, floor heat, etc.). Setting it to ~140 F may not be an option, and anything less than that is legionnairic.
I can't actually remember if it was too hot or too cold (or at some point during troubleshooting), it may have been set too high and was triggering the zone valves too soon, creating an endless loop of not hot enough.
The problem was someone went down there and futzed with the boiler and the spool valve controls, when none of that was the problem, and that put the whole system in disarray costing two days of troubleshooting and a forty dollar part.
We also replaced one of the loop's heat exchangers; don't remember why. That could have been unrelated.