There are 12 different Ancistrocerus species in The Netherlands of which 6 are quite rare.
The other 6 are very difficult (if not impossible) to tell apart from a photo.
However, Ancistrocerus nigricornis can be distinguished by the little "dent" (or hook) at the underside of the abdomen (sternite 2) which is best seen from the side (3rd photo).
In addition, other similar looking potter wasps usually do not fly as early as in April/May but later in the year.
In my yard, Hilversum, The Netherlands, 10 May 2020