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Oops - how embarrassing... For a short moment I considered Opomyzidae, too, but I supposed G. balachowskyi/hackmani to have a more or less homogeneously dark abdomen like the one in https://diptera.i...d_id=14622 or in my added picture from Austria (below). However, in the linked thread Jan Willem mentions a "more striped" appearance of G. balachowskyi, which agrees well. As G. balachowskyi but not G. hackmani is said to occur in Berlin, I hope I found the right name almost certainly - thank you so much!