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Hi Jörg, the ? means, that I cannot count the dorsocentrals to be 100% sure about N. pallida, which should have 7 dc.
The other candidates (quadrifasciata and erichsoni) differ in the abdominal dorsal pattern - this is not clearly visible.
For me it is N. pallida, but I haven't seen enough variants of these to define precisely.

based on: Igor Grichanov (2006): A checklist and keys to North European genera and species of Dolichopodidae (Diptera)