QGIS Print composer scale problems

EPSG:3857 doesn't conserve distances so this will depend on where on the earth you are mapping - I think the scale is calculated at the Equator.

The easiest fix for this is to switch to an equidistant projection like: EPSG:102014 or other depending on your region of interest.

Generally speaking the UTM grids are a good place to start if measurements or scales are involved.

In your particular case (Switzerland) this would be UTM 32N and I'd recommend the ED50 (European Datum) version ED50 / UTM zone 32N (EPSG:23032).

Since Switzerland has its own National Grid system (see swisstopo and epsg.io) CH1903+ / LV95 (EPSG:2056) I personally would use this for any printed output unless I had a specific reason not to.