PyQGIS: Trying to export layer to shapefile

I´m using python in QGIS 2.2 and struggling with an error trying this:

# create layer from text
_vlayer = QgsVectorLayer(_ur1, "raw", "delimitedtext")

works fine but:

# export layer to shape
_writer = QgsVectorFileWriter.writeAsVectorFormat\
(_vlayer,"hoppla.shp","utf-8",None,"ESRI Shapefile")

... ends up wiht error code "3" - according to documentation:"ErrCreateLayer". I really would appriciate a hint.