qgis - Intersection reports invalid geometries

I want to intersect two layers (shapefiles) with QGIS 2.18. For both layers, I tested validity with

• QGIS Vector / Vector Tools / Check validity and


• GRASS v.build.check (with default parameters)

both tests report no errors for both layers, but QGIS Intersection reports invalid geometry for one or more input features.

The two layers may have some boundaries, that should be equal, but may actually have slight differences (unprecise manual digitalisation of one layer) possibly resulting in several very small slices.

What do you recommend me to try?

Answer

Maybe this works (Fixing invalid polygon ..):

https://anitagraser.com/2017/08/29/fixing-invalid-polygon-geometries/