The floor tile in the shower has a white spot

I would almost bet on lime!

yes good we have. But we also have a Grünbeck system. The only strange thing is that it is one spot. we also can't get rid of it

Is this the spot under the showerhead? Maybe the shower drips and this spot always has some water from which the limescale then deposits?

We also have very soft water (6° total hardness). Nevertheless, a stain forms under the shower on our tiles (very similar shade). It comes off well with Silit Bang, but you really need to rinse thoroughly afterwards (preferably also squeegee) otherwise you’ll get a cement film on the tiles because the limescale remover dissolves the grout.

Super! I checked yesterday, it really is under the shower! I'll give it a try.

Report back whether it worked.
Off topic: Hey - you also have a tiled drainage channel. We thought it was very nice too and had it done the same way.