Cracks in the window lintel/clinker - How to act?

Basically, there is no window lintel. The lintel sits above the window and what you probably have is a clinker facade. There is also a masonry lintel there. It has slipped, which is not surprising because it is shoddy workmanship.

The attached lintel has no bearing at all on the side wall. You can see that in the picture. Filling the joints doesn’t make any sense there. The lintel has to be removed and rebuilt. Then with enough bearing on the wall. I’m not surprised it slipped.

From a structural point of view, it only matters for your facade. Certainly not for the wall of the house. It could hold like that for another hundred years, but it might not. I would say it needs to be rebuilt. The effort here is not even that big. Provided you have a good mason.

Here are a few more pictures. The roller shutter is located directly behind the slipped arch.

Basically, there is no window lintel. The lintel is located above the window, and what you probably have is a clinker brick facade. There is also a lintel built there. It has slipped, which is no surprise because it's botched construction. (...).

The masonry lintel transfers the load to the side wall, which is why it does not have to rest anywhere. Moreover, if it did rest, that would be an overdetermination and would cause even more problems.

There is also a lintel built there. It has slipped, which is not surprising because that is shoddy workmanship.

I cannot recognize it. Please explain it to us.
One would want to know how the lintel was constructed (scheitrechter arch according to appearance, but only masonry or precast lintel or with reinforcement).

But in one picture, cracks above a lintel can still be seen. I would actually have a professional come. From a distance, it is hard to assess what the overall damage problem is.

The masonry lintel transfers the force into the side wall, which is why it does not need to rest anywhere. Moreover, if it did rest, that would be an overdetermination and it would certainly cause problems.

How come (and what is an overdetermination)? I still see too little clarity about the construction of the wall and lintel here, only the lintel in the facing shell. That is clearly not professionally executed because this segmental arch is too flat for the stones to "wedge" together and hold each other. The fact that it only slips on one side is actually a small miracle. For repair suggestions, as said, the OP would have to reveal everything regarding the construction.

Some people clearly did not quite understand. As you correctly write, it is a facing formwork made of clinker; one can also say facing shell. Although these are not facing bricks but solid clinker bricks.

The masonry lintel of this formwork is not properly executed, and that is where the damage now comes from. It will probably get worse.

But as explained above, it is the window lintel. It is merely a masonry lintel for the formwork. Statistically, it has no significance for the actual masonry or the window.