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Hi Jürgen,
thank you for answer. I found this old thread
https://diptera.info/forum/viewthread.php?forum_id=5&thread_id=53383
Psilidae I can´t exclude, but Strongylophthalmyia ustulata was the best matching species for me.
I observed the flies on a thick stem of a dead aspen. Most of the Psilidae larvae should be phytophagous on nonwoody plants.
Best regards, Marion