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Angelika/Reimund,
nothing is really difficult as soon as we can see all the details we need to key a species out. The pictures have their limits. To tell you the truth, I am to your side to say it's Eristalis similis, but without seeing clearly the pterostigma, it's hard to prove. This is for sure not E. tenax, and E. interrupta has usually white hind margins on tergits.