QGIS Raster to Polygons produce invalid geometries

I found a possible workaround (inspired by Pavel's reply) by just applying the smooth geometry algorithm from QGIS. So I don't have "square polygons" anymore and this means no ring self intersections.

Thanks

You can use the clean function from GRASS gis that is available in Qgis via processing. In the processing toolbox you may use v.clean with the "break" cleaning tool. It will solve self intersections by adding nodes and splitting the polygon into two. You can also read the help inside the tool for the other cleaning methods. But for this particular problem break will work.