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I believe I have found the species: it is tabanus bovinus, female.

I have just retrieved a key to European horse flies which allows to determine this fly as tabanus bovinus (due to the slim body, the brownish colour, the size, the face marks, the form of the antennae). It closely resembles T autumnalis, which would, however, be larger, thicker and mostly greyish.

By the way, thank you Eric for directing me towards the tabanidae!