Hello @Kreuzschmerzen and welcome to the forum 
This problem is discussed here:
Preview Line for Polygon (and other line) tools
You could try the latest Nightly 5.3.0-prealpha build to see if it’s been fixed yet:
Index of /ci-builds/graphics/krita/master
For Windows, it would be a good idea to use the portable .zip package to avoid making any changes to your installed version.
In any case, make a backup copy of your settings/config files and your resources folder before running it.