Usually this happens when you read form a format that allows multiple geometry types (ex OSM) and write to a format that doesn't (ex. shp).
Hi, @urs, probably you've set "From Schema Definition" to the "Geometry" parameter of the Shapefile writer feature type. If so, try changing the selection to "First Feature Defines Geometry Type".
Just be aware that a Shapefile file can store only features having a geometry of an identical type. It's a restriction of the Esri Shapefile format. Because of this restriction, once you have set "First Feature Defines Geometry Type" to the parameter, the writer rejects a second or later feature (belonging to the same feature type as the first feature) if it has a geometry whose type is different from the first feature.
Alternatively you can set a specific geometry type (e.g. shape_point, shape_line, etc.), if you are sure that the geometry type of every feature is identical.