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EDIT:
Hello Volker, I think it's Abrachyglossum capitatum and I can exclude Leopoldius and the Conops as far as I know the species.
- Arista on top of the 3rd antennal segment
- I think the spikes on the arista are visible.

See: De Bree (2012) ‒ Abrachyglossum capitatum, een nieuwe blaaskopvlieg voor nederland
Nederlandse Faunistische Mededelingen 38 ‒ 2012